Hmmm, under $200... well, I live in S.A. where we earn inferior Rands, so the equivalent of $200 is a s@#$load, so I'll tell you about what I consider great buys, but most are around $100. If you want a knife for defensive purposes you need a fixed blade :
SOG Tsunami, Recondo, Pentagon, Seal 2000
Gerber Mk1, Guardian, Yari
Camillus CQB 1
Cold Steel Safekeepers, Recon Tanto
Benchmade Nimravus
Kershaw Amphibiam
Just love the SOGs, just a pitty the sheathes aren't so great for concealment. I have not had the priviledge of handling a SOG Recondo or a Camillus CQB 1 yet as local dealers haven't had any yet. It is purely due to reading a lot of material that they fit in with my requirements. The Nimravus is somewhere between $100 to $200. If I was in the States I'd be looking at some of those TOPs knives as well that fall in the under $200 category.
Sometimes it isn't practical to carry a fixed blade. On such occassions a good affordable knife like a Spyderco Endura, Delica, Harpy, Native etc or a CRKT KFF or M16 are just great. Most of these can still be found at around $50, which is good value, and you won't need to conduct a funeral for the knife if it is lost in action
