For example, http://portland.craigslist.org/ charges $25.00 to run a "software / qa / dba" ad for a month, but nothing at all to run such an ad at http://spokane.craigslist.org/
It's not really the traffic that counts, but conversions. How much money do _you_ really need? Are you working full-time on this? If not you might be better off to charge little or nothing, so as to build market share.
Most job boards don't do much to market themselves, other than maybe send some spam. There's a lot you could do yourself. Suppose you have a local job board, you could show up to MeetUp groups and the like to promote your board, pass out handbills in public places, send flyers around to student employment offices at universities.
You won't need much traffic at all, if the people doing the hiring are able to find lots of qualified candidates through your board.
This is a highly competitive area. There are bazillions of job boards, however most of them totally suck, so you have the opportunity to do well by doing good.