I am nearly finished setting up an new forum and need to figure out the advertising. The forum is for distributed computing and client monitoring systems. I plan to run a well targeted $100-200 ad campaign on specific sites to get the forum going.

The current plan is to have 6-8 'community sponsors'. I don't like ads on sites, so I am going to try this method, where a company/site pays xx/month for a square in the right margin (somewhere around 80x120px). I estimate the forum to have a couple thousand members within the first 60 days.

What would be a good price range per ad to expect - after the first few months? I will have a discount for the first 3 months due to the growing size of the forum.

Many thanks.

Anyone?

I have no idea. Why don't you monitor forums where people are already advertising space, to get a feel for the going rate?

You could try these places:
Sitepoint Advertising Space For Sale
Webmaster- Sell Ad Space

Thanks for the links. There seems to be a huge range of prices. This is what I am thinking - A Total of 8 community sponsors who each get their own 80x120px 'button' for USD 35/month. The first 3 months will be 15/month due to the growing forum. I estimate a few thousand members within 3 months.