First I would look at the niche, is it something with a lot of monetization options and/or high paying opportunities? If so, I can look at similar sites, I can

First I would look at the niche, is it something with a lot of monetization options and/or high paying opportunities? If so, I can look at similar sites, I can look at PPC bids, affiliate programs/conversions. I could see if competitors with ads are selling and see what sort of price/value they are offering (also checkout their media kits if they offer to see what people are paying). It makes a difference if you're selling a website or a domain alone though. Domain only gets complex because the nature of the traffic matters hugely. Changing things dramatically will lose search engine traffic if that's the major source (toys.com is a good example of this).