My number one rule that I set a few years back when I started seriously collecting was a $100 maximum sales price (Not including shipping). My reasoning was that I'll NEVER buy a 'Safe Queen', and will EDC every knife that I buy. This was after buying and playing with a bunch of low/mid prices autos that I can't EDC legally. I figured that a $100 price limit was a good challange. So far, I've only broken that rule about three or four times, and not by much even then. That means that I don't currently own any CRKs, Striders, Hinderers, or other really high end knives. On the oher hand, the chase for quality knives within that limit has been alot of fun. I have dozens of knives that have street prices well into the mid $100s range that I've found used for under $100.
My other rules are that I only buy folders, I buy knives with a 4" blade or less, and in general I prefer plain blades to serrated or combo blades.