As above, it really depends on what the mix of letters and domains are. Some might mean a lot to most (i.e. 3 letter words or abbreviations) but you can also find that the not so popular are actually very much in demand from only a handful of people to whom they might be very relevant.
I'd also suggest that 3 letter / number .coms are tricky business because, from experience, a lot of people selling them think they should all be valued at the same price as the likes of sex.com mentioned above. When in actual fact this is a completely unrealistic aim.
btw - domain appraisers are scams - don't pay money for services like that. some are legit but 99.9% are scams. there are way too many open-ended and unanswered questions that factor into a domains value.
[edited by: Webwork at 2:21 pm (utc) on Feb. 25, 2008]
[edit reason] Prior domain reference removed so comment removed [/edit]