Super Star Soldier or any game in the Soldier series you don't already have. Aeroblasters.I don't know, is anything on eBay less than $50?
Most of the good ones were already mentioned. Cyber Core and Avenger PCE CD are two shooters that I like and should be under $50.
Can you play CDs? L-dis is decent and can be had on the cheap. I think I paid $12 shipped
hit japan is hit or miss, generally overpriced and if he doesn't like an offer you make he might ban you.
Blazing Lazers is one of my most cherished Turbo games. If you don't have it, I would definitely look there first. It's been going from absurdly low to absurdly high lately. It doesn't make any sense.
Almost forgot Sidearms, pretty great shooter that you can get for 15 if you have some patience, 25 if not.