Cost for banner ads can be ballparked by using a CPM (cost per 1000 impressions) rate method. CPM rates vary but are usually in the $5 - $20 range. The reason for the range is due to how targeted the traffic is. Since you are saying your demographic fits the advertisers target audience, I would lean more toward the higher end of the spectrum.

Now impressions are based on page views not uniques, so you have much higher numbers to use for estimations here but if it were me, and I had just your unique visits to work with then take 2500 multiplied by 30 (since this is a monthly bid), divided by 1000 then multiplied by your CPM Rate ($20) ie: 2500 X 30 \ 1000 * 20 =$1500 per month.

That is what I would charge. Keep in mind, your page views are much higher then your unique visits so that price would be a very conservative price.

Hope that helps,
Dave
HTMLfx.com