You should be able to find a knife to suit your purposes for under $50.
Remember, it doesn't have to be particularly good steel, or hold a good edge very well, if bought for strictly defensive purposes.
For a " working knife ", you may need to up the ante, and get better steel with better edge retention, but not for defense.

I will tell you to stay away from liner-lock folders.
When grasped in certain ways like you might have to do to defend yourself, it is relatively easy to disengage the liner lock and have the blade edge near the hinge close against your hand.
Don't ask me how I know this.

To satisfy your curiosity on this matter, do some full-on forehand and back hand stabbbing drills and slashes on a big piece of heavy cardboard, carpet, or some other decently resistant material. Might want to put some protective tape on your knife hand first though.