If you're talking about a domain name, rather than an established website that is generating a revenue, then your best bet is to take a look at major domain marketplaces (Sedo, Afternic, domainer forums) and see what similar domain names have gone for in the past.
Don't EVER believe any automated valuation tools as none of them are (or can be) even close to accurate. At the end of the day the domain name is worth exactly as much as the buyer that you will find (if you find one) is willing to pay for it, and the only thing that can give you *an idea* of how much it could be worse is past sales.
There was also a site out there that gathers this info from all major domain marketplaces and has a nice search feature but I've forgotten what it's called so google for it!