I've been asked by a lot of people - where can I buy a good quality, but affordable balisong (butterfly knife).
I can tell you with confidence, that the best, based on this criteria are knives made by The One, a Chinese brand that specializes on balisong knives.
I had one of his blades (BM42 version) and I was really impressed by the quality level.
It has channel construction (solid mileed steel handles), spring latch, bronze washers, which enable smooth flip and even comes with a sheath and a torx screwdriver for adjustment.
The knives can be bought here: http://goo.gl/ol5JtG
For a sharp version make the order, but don't pay! Leave the seller a message that you want a sharp version, he will adjust the price (it costs 18$), and then complete the purchase.