One guideline is Sitepoint that facilitates buying and selling of websites. They categorize sites in different categories -- startup, established, premium. One rule of thumb is 10x monthly earnings. I.e., if a site is making $25 a month with AdSense and/or affiliate sales and/or its own product (that comes with the site), then the price would be $250. More or less, depending on what you think the potential is to improve it or not.