I conducted the following poll twice over the years: "How much do you pay each month for your regular aikido classes (in US dollars)?"
http://www.aikiweb.com/polls/results.html?poll_id=286 - 8/27/2005
http://www.aikiweb.com/polls/results.html?poll_id=121 - 7/13/2002

Also, here are some threads on this topic:

check my dojo's monthly fee
How much do you pay?
How much does Aikido cost?
How much does your Dojo Charge?
Average $$$ per pop

Hope that helps,

-- Jun