Alex – I really wouldn’t bother. There are so many ways of doing it you will never get a true value, just a best guess and it will not be accurate. Some value on SEO potential, others on market potential, some by a multiple of current earnings, others rate by sector, length, extension…all sorts. Bottom line is no-one has a clue.
Everyone likens internet property to real estate – but it’s so far removed it’s not even comparable. Real Estate has templates (4 bed house, 1 acre of land etc) so you can place values on the substance. Most domains rely on their “potential” and of course, no-one can put a price on “potential” because it depends on whose hands it’s in.
It’s a cliché but it’s true: a domain is worth what someone will pay for it and that will generally be because of one of two things; Brandability or Keyword ranking potential. The latter is easier to sell as the first needs a marketing budget. But the right brand name could be more valuable if short and snappy or just happens to be what someone needs at that particular moment.
Don’t waste your money. You can get ballpark estimates from some of the forums like dnforum.com or namepros.com by placing it is the relevant forum asking for user opinions, but that’s about the best you’ll get.
One final note – a domain might be “worth” $100,000 but that doesn’t mean to say there will be a buyer prepared to pay it 