No, no and no. Unless you're operating in a market where there's one hell of a premium on your lead payouts, $20 CPM is a recipe for disaster. Even if the market IS lucrative...that's far too much to pay on a buy where you're anything less than certain.
Forums BURN impressions. When you think of the individuals lurking on messageboards, clicking through threads and reading through several pages at a time, that's a lot of impressions wasted on the same uninterested eyeballs.
If a forum is getting 3.5 million impressions per month, it sounds pretty mainstream to me. So go and advertise with other similar forums instead. You should be able to negotiate a much better price with their rivals.
Failing that, use PPV to bid on the URLs. Same targets, cheaper traffic. Make sure you cap your ad views per user so you're not wasting more money.