I have a buddy who owns a website that offers free website templates. He get's around 1500 people per month downloading and using his templates, and each template has a link to his free template site and also a link to whatever site he's currently promoting. He has no ads on his template site, so there is no record of revenue, so I do not know what I should offer. What value should be placed on 1500 natural backlinks per month? I know 1,500 backlinks per month to any website of your choice, with any anchor text of your choice, would be a very nice thing to have, but what is the maximum you would offer for this site? Almost forgot, he spends $0 advertising his website, it's all organic traffic.
You can't really tell. Traffic, search engine positioning, backlinks, traffic, revenue. All of those things have to be taken into account. In the end something is only worth as much as someone is willing to pay.