Repros aren't worth more than $15-30 a piece, Star Fox 2 and a few others are a little different in that they are harder to make. Harder might not be the right word, they have specific requirements.
The "boxes" he is giving you are UGC cases with artwork printed on photo paper and slipped in the sleeve, not unlike a DVD case. I only saw part of the image. Those are boxes with cardboard inserts, that raises the value a bit. $50 might not be bad.
I know plenty of repro producers who will gladly make just about any game for $25-30. Star fox 2 might run you a bit more, maybe $40. But you won't have boxes, inserts, or plastic protectors. Just the cart. Anybody who tells you otherwise is trying to rip you off.
I'd buy Star Fox 2 for $50 if it came with the box.
I'm making so many edits on this post.
The fact that repros are just that... reproductions, there are no limited runs or out of prints, you just produce more, there really is no "market" for these since you can just go find a producer who will make them for you. Yes, if you are going to be a giant douche and throw it up on ebay some moron will pay your price likely, but then you're a douche and they're a moron. If you are buying it for yourself, you just have to make the decision solely on how much you care. Repros don't have much value nor should they. If you just want to play the "hack", just load up an emulator or patch a ROM on a Retron5. If you care about having a fake game on your shelf, then knock yourself out. Repros are definitely not something you should look at wondering if it'll go up in value. They never will.