My favorite at the moment for small, legal, inexpensive, self defense appropriate folders would be some of the Kershaw products by Ken Onion. The reason is due to the quick opening design of these folders; it opens as quick as a spring assisted opener, except they don't use springs so it is legal. For defense reasons, quick opening and quick deployability is actually more important then other factors like design, blade quality, etc. Most folders require some practice for quick deployability (some more then others), but with this quick opening mechanism, little practice is required to have a lightening fast opening knife.
Again, these are the Ken Onion Chive, Scallion, or Leek; lengths and prices vary, but they are within your range. Check out there site:
http://www.kershawknives.com
The one concern you may have is the grip on these knives...they aren't exactly "nonslip." However this is easily remidied with "Griptape", the sandpaper-like tape that goes on the top of skateboards. You can get big sheets at your local skateshop, or online for about $5. You just carefully cut it to the size of your handle, and tape it on. You'll never have a grip problem on any knife again.
I also stand behind "ColdSteel" as a producer of great inexpensive yet quality knives. I'll stand by their design. Cehck out this site at:
http://www.coldsteelstore.com/COL/site/index.cfm?killnav=1
In particular, check out 2.63 inch trail guide, and medium Vaquero for blades under 3.5 inches. I will say that most Cold steel folders are 4 inches in length.
Spyderco also makes good quality blades, but they can be really expensive. A few that are in your range are the Jester, Cricket, Dragonfly, Endura, Kiwi plain handle, Native, Pride, etc. You won't be disappointed with spyderco quality! Site:
http://www.spydercostore.com/SPY/site/index.cfm?killnav=1
I also stand behind Gerber as decent knife company. I would check out the "Gerber International" in particular. I liked the opening mechanism on this one. There are others, but some of the other designs I remember distinctly not buying them because I didn't like the opening mechanism. You can check out there site at:
http://www.gerberblades.com/
I also hear that SOG makes real good quality folders with a quick opening mechanism for decent prices, but I have never handled one. You can get some on discount sometimes from the factory at this site:
http://www.sogknives.com/sog2nds.htm
Hmmm....that's about all I got. I would stand behind all the companies mentioned about as making a good quality product. The rest is up to you to find what fits your needs and personal style!
PAUL