Advertising prices for PR4?

[JosephC]

I own a pagerank 4 call of duty (gaming) forum. I have over 6,000 indexed pages on google, 15k posts, over 1,000 members.

What would be a good price to charge for advertisements? such as a global footer banner ad, under first post banner ad, text ad in global footer / backlink in global footer, etc..

Thanks.