this is a very big question , you can measure domain value by so many different categories , the TLD , the keyword , the market , the age , etc etc
but... in the end of the day , domain is worth whatever the buyer is willing to pay for it , some domains will have no value to you , but to others they can be worth a bundle (this is of course referring to the end users market and not to the resller market where prices are more or less fixed )