How long is a piece of string? What do you mean pre-selling anyway? How many visitors does it get? How relevant are they and to whom? How are the site´s serps? Think you may have to give a bit more background.
offer it to webuildpages and see what they offer, then add 20% or whatever comish they would attach and there is a fair market value.
pre-selling is when the site is not live yet. but i am selling ad space a discounted prices. i can garentee i will get lots of traffic.
Please tell us how you can guarantee lots of traffic and how relevant the traffic will be to the site topic. Not being snipey Bobby9 but if I was a potential advertiser that is what I´d want to know. It´s not only about the quantity of people visiting the site - at least not if you want to sell your advertising space for a reasonable amount of money.
I always do the pre-purchasing of the advertising for my sites that are not designed yet.. often even before I pick a domain or topic. For the time being, before my site goes live, I usually just point the ads at yahoo or google since I like them a lot, I figure the good karma will roll around eventually. Eric