Hi lancsman,
This is just one variable in evaluating a domain, i always check three things, local exacts, reg'd TLD's - what are the Com, net, org, biz, info (CNOBI)and org.uk doing with the domain. And the allinurl.
There are many other parameters to check once the first three boxes are ticked you can look at the other considerations, competition, advertisers, market trends etc etc.
The allinurl is a guide and also shows you which pages ranks highest with that term in the URL, allowing you to review what they are doing - gives you ideas as to what you could do and what not to do. There is no golden rule, but i tend to want to see at least 1 million.
If you want to buy sell a domain then search the exacts and list in ascending order, see if the terms with a few hundred are free to register, i am about to do this in one medium competition area, allinurl over 3 million and i will be hand regging the best free domain and then starting to contact potential buyers. If i get a good bite i hand reg the others and offer the package of 5 or 6 domains to create a nice package, that way i have something that offers a bit more than a passing interest. My 40 to 50 quid investment may make me up to a grand, if i am very lucky, a bit more. If i cannot get a buyer i can throw up a site and see if it earns me my reg costs.
Selling the long tail keyword domains is a good way to "Sell" to endusers, it seems they are getting a lot more for their money and even if i sell them for £300 it is still a good 4 hours well spent.
How to find endusers.....now that is whole new bag of tricks;-)