Rule of Thumb: There are lots of domains that sell for lots of money, like sex, look at that , sex in every extention will sell for above $500 to sometimes even hundreds of thousands of dollars, but just cause that is worth a lot or that word had a sale, doesn't mean that a word like sexxdoctorofthemalaysianmountains.com will sell well. Well I was being funny there, but I'm trying to prove a point.
The value of a domain name comes when the two terms are relevant and make sense of each other, names with mispelling are only good if the original site you are making the typo from gets lots of hits and therefor you get typo traffic that can convert into $'s from clicks.
To really know the value of a domain name you can read my appraisal guide at http://dnfinder.net. Just look for the appraisal guide, it can teach you a lot.