Avoiding being wiped out after work commute? I work ~15 miles from home and have made the decision to make the commute via bike. This is for a couple reasons. First of which being I don't actually have my license yet, second being that taking the bus actually takes longer, and third, that I'm getting my exercise AND commuting done at the same time. 

I was initially doing this on a 45lb+ mountain bike with large tires and no pedal clips, but I recently bought a nice $700 road bike and have cut half an hour off the trip so that it only takes about an hour each way. 

I don't so much mind the time it takes, even though it leaves me with little when I get back, as much as I do how tired I get afterwards. At work isn't too bad because I'm sitting most of the time anyway, but after I get home I've spent 8 hours at work, and two hours riding 30 miles on my bike, and I don't have much energy left to enjoy what remains of my time. 

Diet is, of course, what I'm asking about here. I'm earning a modest pay and trying to put every penny towards college, so I'm not looking to reinvent how I eat, just make some switches or additions here and there. I've tried eating large amounts of pasta the day before, but still didn't see much improvement (I get the feeling I've misunderstood something about carb loading). I would drink more gatorade or other electrolyte-replenishing drinks, but I'm not sure where or how to buy them cheap and in bulk so the cost doesn't add up too much. The other thing I'm concerned about is protein. I'm probably not getting as much of it as I should be, but don't really know what to do about it. I've heard that people who weight train use whey protein - is that something bikers should do as well? If so, how would I go about taking it?

I'm relatively new to biking, so even the simplest advice would be appreciated.
The simplest advice is: just keep it up. Give your body time to adapt to the new workload you're putting on it, and it will get easier and easier.

For drinks, you can buy gatorade powder and mix it yourself at home. $19.49 for 9 gallons of gatorade is way better than paying retail price for a quart at a time.

As I understand carb loading, it's not something you can do for a repeated effort like a daily commute. The idea is to deprive your body of carbohydrates for a period of time, then binge shortly before an endurance effort. You can't really achieve a level that would be useful inside a repeating 24-hour period.

Lastly, don't sweat protein. It probably wouldn't hurt, but any benefit would be extremely minor for the workloads you're doing, and isn't worth significant effort or attention. Again, just keep up with the cycling and your body will quickly begin to adapt automatically.

Edit: Oh, and one thing that will help is to consume carbohydrates immediately after your commute. The human body absorbs carbs significantly more quickly after a workout, and uses them to replenish any lost stores. Waiting even a half hour greatly diminishes this effect, so the sooner the better. It doesn't have to be a full meal or anything; anything is better than nothing.