Cheap cheese is, as others have explained, cheap for a reason.
You should be able to find old/extra old (AKA "sharp"/"extra sharp") cheddar cheese in the cheap section, which makes a reasonably good starting point - this cheese does have some flavour.
Daniel says he simmers the milk; I generally start with evaporated milk, which is even more economical than regular milk and keeps in the pantry forever.
Although I would not use evaporated milk in just any recipe calling for milk, it happens to work quite well for Mac 'n Cheese.
Other common additions to help offset the lack of flavour in poorly-aged cheeses are: Salt.
Remember, salt is essentially a flavour enhancer and will bring out the natural taste of any other ingredient. Mustard.
Prepared mustard is OK, but concentrated mustard made from dry mustard and a small amount of water is even better, so you can avoid having to add too much liquid.
This doesn't enhance the flavour of the cheese, but it does share some of the "sharpness" associated with cheddar and makes a good complement.
As a bonus, it also acts as a natural emulsifier for the sauce, helping to minimize separation and curdling.
Ground cayenne or red pepper.
Although most people probably don't associate Mac 'n
Cheese with piquant , a small amount of this won't make the sauce noticeably spicy , but it will add a bit of the same "kick" you get with very old cheese.
If all else fails, my "secret weapon" for Mac 'n Cheese is - not kidding - the sauce base from Kraft Dinner (AKA Kraft Macaroni and Cheese Dinner for you Yankees), especially if you can get the "extra sharp" version.
This stuff is dirt cheap (often just 99 cents for a package) and before you dismiss this as heresy, keep in mind that the idea is not to use the whole thing .
Just a teaspoon or two to enhance your homemade sauce.
If you don't approve of such shortcuts then you shouldn't be buying cheap cheese in the first place,
right?
So there you have it.
Try some or all of the above
; I'm sure you'll end up with something that's at least acceptable, if not great.
cooking.stackexchange.com/users/41/Aaronut
You would need to add other flavours; you can't add cheese flavour without adding more (or better) cheese.
When I make M&C, I like to simmer the milk before adding it to the roux, and I poach a clove or two of garlic in the flavour.
Adding well-rendered bacon is also nice.
cooking.stackexchange.com/users/None/
The biggest difference between expensive cheese and cheap cheese is time.
A cheap block of cheddar aged in the fridge will become more and more sharp as the bacteria continue to work in it turning lactose into lactic acid.
This takes up fridge space and time and you have to carefully remove any mold (or reapply a rind before you start).
Just for the hassle
it's probably not worth it.
This is a big part of why good cheese costs more.
For example, the manufacturers of the inedible canned "parmesan" powder that is sold in the USA have lobbied the FDA down to 6-months of aging to save costs.
I would recommend adding non-cheese flavor additions as Daniel suggests.
cooking.stackexchange.com/users/2001/Sobachatina
You could go halfway and buy a small bit of really sharp cheese and mix it with your cheap cheese.
Do you do a crumble topping?
I find that the crispy texture can add an extra something to Mac and Cheese, as well as additional flavors.
cooking.stackexchange.com/users/1675/Manako
make your own cheese, it will be tastier and cheaper.
its pretty simple, everyones favourtie search engine will give you a good list of result on how to do it.
cooking.stackexchange.com/users/1160/NimChimpsky
I'd suggest the downsizing method I use for most things.
Start with the expensive option and slowly downsize until you reach the point where you can taste the difference, then go back one step.
This works for recipes and products.
You could start with your original cheese and then slowly replace this with cheap cheddar until it makes a difference.
My mac'n'cheese recipe uses half gruyere and half cheap cheddar and turns out really well at a reasonable price. :-)
cooking.stackexchange.com/users/4185/zeedo
Yes, any aged cheese would be better than plain cheap cheese.
I tend to make mac 'n cheese when I have a selection of cheese starting to go bad.
I mix it all up in a roux, stir in the noodles, sea salt and fresh ground black pepper, a little mustard and a little cayanne or hot pepper sauce.
But my secret to really yummy mac 'n cheese - the "adult" version - is a splash of white truffle oil.
Sounds terribly expensive and decadent
I know
but I found a large bottle in Winners - or equivalent - Ross Dress for Less for about $5!
It's potent, and packed with flavor
and you need just a drop or two
so it will go a long way.
It's yummy in any noodles, on home-made pizza, etc.
A breadcrumb topping is also a must if you are looking for flavor.
cooking.stackexchange.com/users/9335/Karen
Ways to add flavor to Home-Made Mac and Cheese; Add Different Types Of Chopped Pickles; Sweet, Dill Etc.
Add Different Types Of Mustard; Honey, Reg.
Etc.
Add Different Types Of Ketchup; Sweet, Reg.
Etc.
Add Different Types Of Hot Sauce or Hot Peppers.
BBQ Add Different Types Of Pasta Sauces, Tomato Sauces, Pesto.
Add Different - Varies Types of Vegetables, Meat, Fish or Tofu.
Add Different Types Of Herbs, Spices and Seasonings.
cooking.stackexchange.com/users/15452/Leigh
when I used to use a mild cheddar for economic reasons, I would add a pinch of dry mustard to the grated cheese and mix it up.
It gave the cheese a sharper flavor.
cooking.stackexchange.com/users/18218/valerie
I used to make mac and cheese with cheddar.  Like many, I'm cutting costs due to the recession and I'm buying the cheapest cheese I can find.  The problem is, when I make macaroni with it, it's just tasteles.  Is there anything I can do to keep using the cheap cheese, but get more taste out of it?
Cheap cheese is tasteless. Can I make it tastier in my mac and cheese?
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Sharp or mature cheese from the cheap section will have a bit more taste, or you could use cheese aged in the fridge that will have more lactic acid. It may also be tastier and cheaper to make your own cheese. You can also simmer the milk with garlic before adding it to the roux, or use evaporated milk. Mustard or cayenne pepper will add flavour, as well as other hebs and spices, Kraft mac and cheese seasoning, pickles hot sauce and other ingredients. Try letting your cheese age first or make you own cheese. You can also add spices like mustard, cayenne pepper or hot sauce. Try simmering the milk for the sauce.
Look for an aged cheese in the cheap section or leave your cheese to age in your fridge. You can also make your own cheese. Try adding spices like salt, mustard powder, ketchup, hot sauce or pesto. Try simmering the milk or using evaporated milk for more flavour.