My observation, when I was still buying books, is that much depends on the print runs at the time the books were popular. I have first editions of Bruce Catton's trilogies. Problem is, everyone else has them too. They're cheap.
There's a book out there on the battles leading to Nelson's defeat at Richmond, KY. When it was printed, there was little interest. Now that there is interest, it is difficult to find, let alone purchase.
When I was buying books, I bought almost exclusively from abebooks.com. If you can't find a book there, chances are it will be very expensive if you do find it elsewhere. Even "reading copies" are scarce. (Reading Copy is a grade which means only that there are no missing pages.)
Supply and demand. When the demand exceeds supply, publishers like Konecky will pick up the copyright and issue another edition. Apparently, the Richmond book has not attracted enough demand to republish; therefore, it is rare and those who would like to have it must think about the arm and a leg thingy.