Currently the company I work for owns the .co.uk of our domain name - an individual owns the .com version. We've asked him if we could buy the domain as he's only really using it for personal photos etc and he said he'd only be interested in selling for X amount. Is there a governing body or place we can go to that will indicate how much the domain is actually worth on the open market so we know if we're getting a reasonable price?
If you do not have the experience to do it yourself you can hire a 3rd party expert to do a domain appraisal for you and issue a complete report that will indicate the exact value. You can PM me for further details, I can do that for you.