Evan Prodromou just published a great essay on paying wiki contributors. He says don’t, offering solid reasons and alternatives. One alternative that I won’t argue with (but probably one of the least interesting–read the essay for more):
Donate. Set aside a good part of the profits from the site (if there are any…) to donations to related charities. Donations to Creative Commons, the Free Software Foundation, and Wikimedia Foundation are probably all good candidates. There may also be domain-specific charities you can contribute to; if you have a site about pets, say, you could contribute to the Animal Rescue Network.
Relatedly, Evan’s talk at SXSW this spring on Commercialization of Wikis was the best session at SXSW (says me) and a must read for anyone building a community or user generated content site (Evan argues that many such sites have “WikiNature” though they aren’t formally wikis).
To wrap up the self-serving nature of this post, Evan recommends CC licensing for commercial wikis. But that should be obvious.