Anthony, I've had the same thing. I had the same weight from 17 years to 20 years. It just stayed at 70 kg, and still was untill about 9 months I started eating more! I know you've read that a thousand times already, but it's as easy as that
I started eating one more meal a day (or even more somethimes) and in the beginning it was pretty hard, but after a week or two I got used to it and I find myself hungry most of the time

(what wasn't the case earlier) and now I weigh 78-79 kg and I'm looking to add even more the times to come.
I say you try eating more instead of buying those supplements, as as you mentioned, you're broke and in college, so some extra food will be less expensive than supplements (imo)