I would suggest the Clackamas County Fair and Events center. This is an incredible facility with enough space for very large events and small settings inside for smaller more intimate events.
The management at this facility is exceptional and the prices are reasonable. I highly recommend them!
http://www.clackamascountyeventcenter.com/default.html
It might be reasonable but I don't think I would have much of a crowd show up...
I have to say since I wish Multnomah County would move out there. They had their last fair at Oaks Park and that was really kind of a joke. I love oaks park, but I don't need to see salesman between every ride.