Here are a few numbers to have in mind.
Communispace charges a whopping $15,000 – $20,000 per month for communities of 300 – 500 people – mostly market research projects. Great work, if you cna get it.
Some companies charge per day, e.g. moderation for £200 – £400 per day. This doesn't include developing your online community and driving interactions etc…
Companies like Telligent and Jive Software offer a premium platform packed with analytics, hosting and excellent features/support for around the $15 to $40k per year mark. This doesn't include active management.
Some companies charge by a cut of the value generated e.g. 30% of all affiliate sales from the community. This is risky (the business might not give you much support). Not recommended, but if you decide this charge a high %.
Salaries average around the $75k – $85k mark (depending if you're male/female, surprisingly).
My retainer-based fees generally range from £4.5k – £9k per month, non-inclusive of expenses and costs involved with the community (e.g. hosting events, developing a website etc..).
The range is determined upon a broad criteria. Namely how hard it will be to develop the community, what the value of the community is (e.g. I'd charge more to build an online community for billionaires than a community for a local gym club) and the level of support being offered to develop the community.