Among the manual flippers that are made in USA for less than $50, your only real choices are the Kershaw Skyline and the Buck Vantage. The problem with the Buck; I've never seen one that was a good flipper and most that I've seen from the Select and Avid line have had fit and finish problems. Between the two, I'll take the Skyline. It's an incredible knife at this pricepoint.
If you're looking for Assisted Open USA made, you have a few more options. Kershaw offers the Link (my favorite from this group) and all the Onion designs (Scallion, Leek, Chive and Shallot). Buck has a couple of offerings that I don't know much about; the Inertia and the Rush. If you can stretch the budget a bit Kershaw has the Knockout, Piston (currently discontinued but still available), the Rake, the Camber and a myriad of different Blurs. In my opinion the Knockout, Piston, Rake, Camber and Blur are all worth the little extra money over the cheaper AO knives. You might be happier to save a bit longer and go for one of them.