This is an excelent thread on a very important topic, and I'm glad that ajri brought this up. I hate to post what others have already, but at the risk of being redundant, here it goes. While folders have their place, personally I feel that a fixed blade is the ne plus ultra in a self defense blade. Sure, you can get a folder out pretty quick if you practice enough, as well as use it as an impact weapon if you can't get the blade out in time ( you should also train/carry an impact device in conjunction with the knife, IMHO) but a fixed blade, even a small one, can be drawn and put into action
instantly. There are too many to go into detail about here, but the HideAwayKnife(invented by a woman,FYI) should fit into your price range with one of the basic models, as well as a vast array of other "neck knives". I have a few small fixed blades that I rotate on a frequent basis, my favorite being a type of
kiradashi, or Japanese marking knife, 3 3/4" overall, with a 1" bevleled blade. Last time I read California Penal Code (been a while I admit) this is not illegal carried on your person concealed, and it is very sheeple friendly: used it in public not long ago and did'nt get any strange looks. About the only other suggestion I can put out there is to have the knife/ impact weapon/ pepper spray on your person in a position that it can be accessed by
both hands, and that you've trained abidextriously enough that you are capable of utilizing it as such. Just my two cents. Here's a pic of the kiradashi: