I would take your page impressions and multiply them by a reasonable CPM rate.
Say you get a million page impressions per month and a reasonable CPM rate -- for a single ad in that spot - in your industry is $1.That means you should charge $1,000/mo for the ad.
If they want good ad positioning, they should pay more. If they want to pay less, they should be OK with footer links. CPM changes radically with page location.
Then, you normally offer higher rates for shorter-term ads and discounts for longer-term ads.