For everyday factory-made knives, I generally stay under $100. Sweet spot for recent orders has been $50 and under for Boker traditionals, under $75 for a Ka-Bar Becker, and $20-$80 for various Scandinavian brands.
Something mass-produced but uncommonly attractive, or a desired collectible, I'm willing to spend more. Recently bought a NIB uncommon Old Timer for more than I'd pay for a current-production knife. There's a new version Bark River Tanto calling my name, but with a $100 price increase of the old version it's not likely to happen. There are a couple of nice fixed blades in the $150 range that I'd definitely consider.
Anything more than $150 and I'm totally not interested in a production-line knife.
For a custom, so far the limit has been $225 if I recall correctly. I could spend more than that, but not without some serious pondering.