Many of the knives suggested are very good. Here's my two cents:
My personal favorite under 50 bucks is the Benchmade Ascent. When I bought it I chose it over the Spyderco Endura simply because the Ascent fit my hand more comfortably. I have purchased the Endura since then, but I still carry the Ascent WAY more often.
I just got a Spyderco Lightweight Calypso Jr (plain edged) today and I think it might be my new favorite under your $50 limit. I bought mine for $43.00, and it think with the full flat-grind and VG 10 steel, you couldn't do better for the money.
If you have a local knife dealer around, I suggest you go to the store and look at all of the ones you're considering. Hold them in as many different hand postitions as you can think of, clip them to your pocket/waistband, grip them really tightly and see if any of them hurt your hand. For example, when I hold the Native tip down, with the spine forward, and sqeeze tight, my hand disengages the lock bar. Eveyone has different personal tastes and hand sizes so you might be different, but I don't want to carry something that might not be totally safe. I suggest you try as many as possible so you find what suits YOU the best. Good luck!