Advertising on a gaming forum? Somehow that doesn't sound right to me. You're not an advertisement forum, so it shouldn't be treated like one. I would find it confusing if I even started charging for custom ads. It would probably make people wonder what sort of forum you really are.
First off if you are selling text links I'd suggest that you use the no-follow tag, otherwise you might find your site penalised in Google. I'd suggest starting off with the following prices per month;
Footer text link - $3
728x90 image in footer - $7.50
Text link under first post of thread - $2
728x90 image under first post of thread - $10.00
88x31 image in footer - $4
These are just very quick guideline prices off the top of my head. Once you start offering ad slots you can increase your prices if demand is high and reduce them if demand is lower than you'd hoped for.
Good luck with selling them.
Edit: Another suggestion - if I was you I wouldn't start selling all those ad slots at once. I'd start off with two or three different slots, perhaps one graphic ad offer and one text link offer and see where it goes from there.
