How can you work out how much a site is worth ?

[advaita]

I have just registered a name - organicfertilizers co uk - and I am trying to figure out how much the .com of the same name might be worth. The .com transfers you to a developed lawn fertilizer site - is there anyway you might be able to figure out how much that site might be worth and therefore get an idea of how much the .co.uk might be worth ?