food is key, and should always be prioritized. this will include protein supplements if you use them, and should definitely include workout nutrition.
caffeine is cheap, and available in several forms, from energy drinks to pills. caffeine pills are dirt cheap.
the only supplement i really value that hasn't been mentioned yet is taurine. This really depends on a person's individual needs, but I find it to be EXTREMELY valuable to me when I do intense workouts, particularly ones that involve my lower back significantly. It makes high rep squatting much more tolerable by reducing back pumps. If you don't workout intensely, this is not such an important supplement. But the price is great. I got a year's supply for like 12 dollars. 1 dollar a month is tough to beat, lol.
creatine is good, and again it's very cheap.