A domain is worth money because of several things. One is shortness/ease of use, and the other is existing backlinks/PR.

I've devised a short test, in which you 'score' the domain from 1 to 100.

1. Is it pronouncable? If so, score yourself 10 points.
2. Is it short/not overly spammy? If so, score yourself 15 points.
3. Does it have useful keywords? If so, score yourself 25 points.
4. Does it have PR? If the PR is between 1 and 3, add a point, if it's between 4 and 6, add 5 points. If it's between 7 and 10, add 10 points.
5. Is it in the DMOZ? If so, add 10 points.
6. How many backlinks does it have? Add backlinks/100 (max of 10)
7. Is it listed on page 2 or page 1 for some keywords? If so, add 10 points.
8. Is it on wikipedia? If so, add 10 points.
BONUS: Is it old? Multiply the # of years by 2, and add those as points.

Now, 'grade' your domain like this:
100+: A++ (Might go for over $2500)
95-100: A+ (Easily over $2000 or more.)
85-95: A (Probably around $1500)
80-85: A- (Probably around $1000)
75-80: B+ ($850 is a good bet)
65-75: B ($650 might be ok)
60-65: B- ($500 is a good deal)
55-60: C+ (You could get $450)
45-55: C (Maybe around $200)
40-45: C- (Not higher than $150)
35-40: D+ ($75 is all you'll get)
25-35: D ($50, maximum)
20-25: D- (At least $25)
< 20: F (RegFee or worse.)

Don't like the payout? Improve it by fixing your problems! Try to get an A or better, you can do it! Be sure not to spam though.

Disclaimer: This is meant as a guide to increase your domain worth (by trying to raise your score), and is NOT something you should go by when purchasing/selling your domain. I make no guarantees that the domain is actually worth that much/that little. You should instead ask the buyer/seller how much they are willing to go for, if you have little experience. Be sure to ask around, to be sure you don't get ripped off. Anyone using this guide as basis for a sale is not very bright.