My favorite LOW COST folder - the one I bought for my kids when I assumed that they'd eventually lose them - is the Kabar Dozier Folding Spear. I don't think the Dozier Folding Spear is currently available, but the Kabar Dozier Folding Hunter is very similar.

A 3" locking folder for a bit over $20!!

My son used that knife from Cub Scouts (when introduced to knife safety) until he aged out with his Eagle rank at 18 years of age. It has served him well.