My opinion on knives is that if you're buying something strictly for self-defense, just about anything will do. The only real requirements are that it can be opened quickly with one hand and that it locks open securely. Sure, it needs to be able to take an edge, but it doesn't have to keep one, and just about any blade will do that. Also, it has to be strong enough that it won't snap, but again that's just about anything.
Now, if you actually plan on using the knife day in and day out as a tool, then you care more about quality. Not because it affects the utility of the knife as a self-defense tool, but because it affects how well it works for all of the other stuff.
Personally, I rarely have need of a knife as a tool, so I carry a 3.5" folding blade that I bought at a gun show. I paid $5 for three knives (together, not $5 each). It's compact enough to fit nicely in my pocket, mean-looking enough that it should be intimidating and sturdy enough that it won't break under the very occasional use I give it.