I've searched the net without much luck. Usually the auction sites and recent sales can give you an idea of market value but this 640-1 is different and I'm not having much luck. It's an all stainless Lew Horton Quadport, manufactured in 1999 and the owner estimates it to have less than 50 rounds through it. The Trijicon front sight has a tritium insert that is all but dead, and there are two long ports on either side of the barrel next to the rib.

Anyone got any ideas, or where to get an estimate?

Here's a pic of a quadport. It's about 8 guns down, on the left side.
Lew's #1 Serial Number Guns