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explain
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You want to buy a tractor. It's 50000$. You're planning on getting it, but there's no urgency. You're at home and get a notification that the tractor is on sale for 49,951$. Do you drive 5 minutes up to the store to get it? Probably not.

I don't understand. If I have the option to spend $50000 or $49951, and the latter only requires a 5 minute drive, then of course I would take the latter.
Of course, in real life, the sale on the hat is unlikely to last long - either because the inventory will be bought, or the store isn't particularly keen on running a 98% off sale in perpetuity. Similarly, the tractor is likely to have larger sales in the future if they aren't all sold. Those factors would make a real life person more likely to go get the hat now and wait on the tractor, but this hypothetical isn't intended to take future pricing into account. It's purely a hypothetical that assumes you get a $49 discount if you drive 5 minutes. >

Okay, hypothetical 2 is probably what I should have led with:
so you go to the store to buy a tractor for 50k and suddenly its 10$ more. do you buy it?
now u go to store to buy a 50c gum and its suddenly 10$ more. do you buy it? >

I maybe buy the tractor, I don't buy the gum. However, this hypothetical is asking a different question than the hypothetical in the OP.
It's likely that somebody values gum more than 50 cents, but less than $10.50. It's unlikely that a person values a tractor more than $50000, but less than $50010.
The original hypothetical assumes that you are willing to purchase the items at their original prices, and lowering the price from there obviously doesn't make a rational consumer unwilling to purchase them at the new price. >

This is probably why the original example is also only $2k and not $20k. When these small increments don't make a dent, the value isnt perceived differently. You can probably swing several hundred dollars in either direction for a $50k tractor or car before someone changes their mind. But even at $2k, someone might jump at $10 off its something they were already going to buy anyway. >

The point of the original example is that it's irrational to not perceive the values differently. >

Right I understand that. But the point is also that humans aren't necessarily rational. 
My point was agree with yours about the original price point. I genuinely can't remember if the car I want is listed at $49,999 or $49,900 or even $49,499. They're all basically the same. They're all "about $50k". So another $49 one way or the other doesn't matter. I value the original price of that vehicle somewhere between $49k and $51k. 
But that swing doesn't hold true for gum.
I think another issue is in the frequency of purchase. Both a phone and TV are things you don't purchase that often, so sure, you should be looking at those $10 the same. But a hat vs a tractor? I will probably buy 10 hats (or if you're multiple husband, 50 hats) by the time I buy a second tractor. So you can't treat these the same in terms of perceived value. I can also pick from thousands of other hats if this one didn't work out, but my options for tractors are way more limited. >

I think I read the same textbook. The example wasn't about "running out to buy something on sale." It was about whether or not to go to the store across town instead of the one near you. 
If you're already planning to buy the item, and the only question is "is it worth the extra gas money to drive across town to save $10," then it really doesn't matter if the item is expensive or not. 
Your extra examples weren't relevant, because in both cases you're spending money you don't need to be spending. >

I put the exact situation in the first part of the post. yes.

Your extra examples weren't relevant, because in both cases you're spending money you don't need to be spending.

you will be spending the money eventually... >

My point is, these are entirely different situations. In the first examples, you've already decided to buy it today, and the only question is where to buy it.
In the second example, you haven't already decided to buy it today; so when you see the sale, you have to decide if it's worth changing your plans, making arrangements for the money, etc. >

okay then use the tractor-gum analogy i gave elsewhere: you go to buy tractor. its 10$ more than normal. do u buy? now go buy a 50c gum and same thing. do u buy? >

That's still not the same thing. Deciding whether to buy something on sale really doesn't have anything to do with cost-benefit analysis. The textbook example was about whether or not to spend resources driving in order to take advantage of the sale. 
Please have your professor explain this to you. This is only Chapter 1. >

or how about you explain why this distinction matters. >

If you have decided you will buy today, all that's left to do is find the best deal. Once all possible deals are on the table, you can cost/benefit analyse them to see where you buy.
If you want to buy something eventually, but not necessarily today, then looking at all offers available today doesn't provide all possible deals. You don't have the constraint of buying today, so there's the possibility of a better deal arriving tomorrow, or the desire of buying disappearing in the future. You don't have to make a choice among the currently available offers, so there's more to it than selecting the best from a list. >

But don’t you also have the possibility that the deals you have today won’t be available tomorrow/go higher? I often face this buying pressure when investing. I don’t need to buy today, but also don’t want to miss out on gains. Quite the pickle. >

That sort of stuff gets abstracted away in thought experiments like this. >


In both cases, you're saving 49$, but clearly one is more compelling than the other. I don't think this is irrational. I think it's an appropriate assessment of the change in value

You haven't actually provided evidence for that, or challenged the original example, you just repeated that this is what people would intuitively do.
Which is the point of the example. People are irrational. They would save $10 on the phone but not save the exact same amount on the TV.
They would save $49 on the hat, but they wouldn't bother to save the exact same amount on the tractor for the exact same effort, because they care more about the perceived feeling of value, than about the actual tangible amount. >

I think the problem is you're changing a lot more than you intend to in your example. In both cases, you add the caveat "you're planning on getting it but there's no urgency". But this really doesn't capture the actual differences between buying a hat and a tractor. You can't just swing by and get a tractor and stash in your closet until you need it. There's just no way getting around that the act of acquiring a tractor is a much bigger deal logistically than getting a hat. So you're weighing that cost against the $49 dollars of savings, which is why you get your result. This tradeoff isn't present in the economics textbook.
A better example would be if you get a $49 discount by preordering either the hat or the tractor, but the pre-order process requires spending an hour to fill out a survey. In this case, it makes no sense to fill out the survey for the hat but not the tractor. In both cases, it's the same trade of $49 for an hour of your time. This is a lot closer to the example in the economics book. >

i fully agree with what you're saying here. thats my position. basically the textbook example is very specifically restrictive and isolated in its setup, whereas i can easily find examples where the abstracted principle doesnt make sense outside this restricted set of conditions. >


basically the textbook example is very specifically restrictive and isolated in its setup

That's because the textbook was trying to teach you a concept. Physics examples are usually also specifically restrictive and isolated (no air resistance, no friction, spinning of the earth doesn't matter, gravitational pull of the moon doesn't matter, etc.)
Easier to learn the concept being taught when you're not being distracted by several other things that factor into the calculation.
You're saying the equivalent of "Obviously gravity doesn't accelerate all things equally, I could drop a feather and a bowling ball and the ball would drop first. The textbook examples of two cannonballs of different weights or the feather and hammer on the moon are just specifically restrictive and isolated in its setup." >

yes >


You're saying the equivalent of "Obviously gravity doesn't accelerate all things equally, I could drop a feather and a bowling ball and the ball would drop first. 

Okay but you understand that this is wrong, right?