I have a forum that sees about 50K page impressions per month and am considering selling a few banner spots.  I'd be interested to hear people's ideas on banner impression pricing.  The site is PR3.  I have seen all sorts of pricing structures across the web from $7 for 100K impression to $1 per 1000 impressions.  How much should one charge per 1000 impressions and where would be the best place to solicit offers?