How much is a Domain name Worth?

That really depends on a lot...

How long have the domains being registered?
How much traffic has the domain got?
Are you also selling it with the site design?

These are just a few. Just ask yourself this "when you are going to buy anything, what type of things do you evaluate?..That is the same for a domain, in this case the evaluation is "How much traffic can I get which will convert to $"....

Most domains which IMers register are usually reg fee, since they're great for development and all, but haven't got a very high reseller value.

But yet, it's impossible to say. If you have "BadBreath.com" (which I assume you don't ) it'd be worth high $xx,xxx to low $xxx,xxxx

If you have (say) Boat.com it'd be worth low-mid $x,xxx,xxx

If you have a 2-4 word keyword domain and the keywords don't make massive sense, the domain may not be worth much.

But to be honest if you plan on developing it, the domain's value doesn't really matter. You can have a 3 word keyword domain worth $0, but develop it into a micro niche website that makes $4+ per day. So the domain would be worthless, but the website/development of it would be worth a lot

Edit: To clarify, my above post is going solely on the basic reseller value of the domain name (as per the OP). Although as mentioned by the other posters, if you sell the domain with the website (which is then more of a website sale) you'd obviously go on the value of the website plus the value of the domain (if it has a value)

Thank you everyone, your advice are noted and helpful.