I'm going to stick my neck out here, and say that appraisals here will be all over the place, but most people appraise based on intra-industry wholesale prices... which can be pitiful, compared with what you can get from somebody truly in the industry the domain represents. The difference is that sales are not as easily achieved to these "end users" and many people price based on "flipping" a domain. Somebody goes into a wanted thread and says (metaphorically) "I want a two letter .com with a page rank of 1 making at least $10,000 a month, and will pay three months revenue." and disproportionately to the out of step requests that are sometimes made, some people who are naive, end up selling. It might not be that high profile, but it might have a lot going for it that makes it a $xxK name, and they end up selling it for $xxx. So you have to know what YOU think it is worth, and why, (value of a lead in the industry, etc.) and be willing to wait for that, or you can approach it with a high volume, low price approach, and then appraisals and sales within the domain community often reflect that.
The flip side of that is people who think that a domain is worth $20M, and you see those people on ebay all the time. It would be nice if market forces would balance requests/sales out a little better, but the industry is still fairly new to most of the world. In fact, most people who are naive about the whole domain/advertising thing don't see why they should pay any more than a little over reg fee, and so it's all ... very interesting, and will require a lot of patience and persistence in development, etc.