It's cost me as little as possible, all my knives have been excellent deals or finds picked up at fleamarkets, yardsales and estate sales, one of the reasons I collect vintage traditional slipjoints is that they can still be found in collectible/usable condition for a reasonable price if ya hunt around, (at least here in North East PA one of the birth places of the knife industry).
One other thing is this addiction/hobby's been spread out over 35 years so for the most part the cost has been spread out.
I do buy the occasional custom and that's where it can get costly, than again there are a lot of customs on the secondary market that can be had at a savings.
One final thought, there are collectors, people addicted to the wow/gadget factor and accumulators, (like myself), pick a pattern, style scale material or whatever, just try to build a collection and don't just accumulate knives.