Page Rank is not that crucial, as it can be taken away if selling/buying just a domain (as the content is removed when the domain is sold, google doesn't connect the content to the domain and the page rank will drop).
What I suggest for you to do, is to use valuate.com as a base estimate. If it shows 10$ for a domain that has a plethora of backlinks from legit websites, then there is something black hat about it. If it shows you something above 100$ you should be fine.
After that the best procedure is to try and find out if the domain has been sold before, and for what price. You can usually bump up the price a little since it is now older than it was before.
.com is usually approximately 60% more valuable than a .net, and .org is 80% less valuable than a .com (Rough approximations, but these give guidance.)
