I disagree. Spam has a low success rate, so it's only extremely profitable when the cost per advertisement is zero. If you have a service where it costs _any_ money to post then spammers will go find somewhere else.

I think it could work well for classified ads since that's something people are willing to pay for. Not so well for wikipedia or social news, because who cares enough to pay money for those?

I'd price classified ads at one dollar each -- small enough that it's practically free, but large enough that you could easily make a lot of money out of it if people ever decide to abandon craigslist.