[jppope]

Use Spyfu, SEMrush, or Google's keyword planning tool to see how much they charge for ads on keywords related to your content. Since you know the traffic (and hopefully your SERPs) you can estimate the total traffic for your site and competitor sites. This will give you a feel for the market. Reach out to the owners of comparable sites (not your competitors) to see how much they are making in ad revenue (Aka if you do shoes, talk to the people that do boots)

Using all of these supplementary data sources determine the possible annual revenue of the site (within a range of outcomes). Take that number and value it like a business (Revenue, costs, etc) and charge a reasonable multiple for what it would earn the new owner in the next 3-5 years (3x - 5x ARR is standard for service businesses, products can be higher). I would also recommend adding a 20%-40% increase on whatever your number is to aid in your negotiations.

Good Luck