My staple supplements are...
10 lbs bag of a protein blend, ~$100
Giant bottle of fish oil, ~$25
Vitamin D, cheap as hell
Creatine Monohydrate, $12
Thats about it, and most of those last several months, if not longer.
Occasionally I will try something weird. Such as unflavored free form beta-alanine and citrulline that I have been throwing in the pre workout protein shake. But, again, these are like $15 for 250+ servings, hardly even worth noting in the budget. I think unflavored free form is the way to go, cheap as hell and you can dose it how you want. If you want to flavor it, those flavorings in the grocery store work well.
Also, I have gotten back to taking Rhodiola again, which is a very underrated supplement in my book.