1. I've got see myself using the knife. I don't need much of an excuse, but I at least have to imagine situations where I'd use it.
2. How much can I stomach to lose? Accidents are going to happen in the kitchen. Three hundred dollars seems to be my sweet spot. More expensive knives will have better fit and finish, handles, and might cut five percent better. I'd always have second thoughts about using an expensive knive.
3. Trying knives from different makers.
4. Not taking forum advice, and trying a different knife. When I picked up a Honesuki, the forum wisdom, was there wasn't much use for it. I de-bone a lot of chicken parts, and the Honesuki does a good job. A necessary knife? No, but I found I prefer it over a petty as a general utility knife.
So what guides your decisions?