I’m a Mayor and our city joined the area Chamber (suburban Mpls-St. Paul) a couple years ago. It’s a very small operation, but I’ve seen mostly bankers / mortgage types and others at the meetings so far. However, I have noticed a couple of consultants (e.g. communications, desktop publishing types) and they seem to do a pretty good job of networking. I’m not sure how much business they get – but I suspect that part of the decision to join rests on whether the Chamber members themselves are folks you see as prospects for work, or whether having membership would get you some PR, credibility, etc.
I should mention that our local chamber just set up a new lower rate structure for ‘home-based’ businesses to try to make it more affordable for consultants, sole proprietors, etc. I’m not sure how much it costs per year though…here is the link to our chamber.