There are no hard and fast rules about valuation. However, in general, for business related terms, the dotcom version
will be the most valuable and is usually already registered. Non-profits are the preferred customer for dotorg and will usually not pay as much as a business will for dotcoms. Dotnets fall somewhere in between. But again, these are only generalizations as a recent hi-value sale of mortgage.org to a bank illustrates.
The most important rule to remember is that "a domain is worth whatever someone is willing to pay for it"
Appraisals are just "an opinion" of what the domain market is paying at the moment for "similar" names.
That doesn't mean that you will find a buyer willing to pay that amount in the next month or the next year.
Likewise, it doesn't mean that you won't find a buyer willing to pay 10 times that amount today!
Here are some domains sales that "break the rules' to illustrate my point:
sexe.org 151,400 USD 2008-06-17
playgroundequipment.info 10,000 USD 2010-09-23
tennesseeanytime.org 16,865 USD 2013-11-13
I do my own appraisals for 2 reasons:
(1)To help decide how much I should pay for a domain to give me a good chance of making a profit when I sell it.
(2)To help decide whether I should sell a domain when someone makes me an offer.
Note that I said "help decide". My appraisal is ONLY one of my criteria. Others include my assessment of the buyer or seller, my individual interest in the domain, my financial situation, etc.