Consider the amount of time, work and effort you've put into it, but in the end list down the items and points that are good about the site (for example, traffic, revenue if any, members if a community, active users, goodwill if any, cost in design and development and marketing, how important it is to the buyer, etc.) all of which don't make for a standardized valuation model but which are important factors in justifying your price, both to the seller, but also to you as well in assessing and making an inventory of your assets, both tangible and intangible ones.