Dan Wesson
It was originally a Dan Wesson revolver, evidenced by the distinctive hammer shape and cylinder release latch on the crane.
It has been re-barreled and the rib sight arrangement mounted for PPC (Police Pistol Combat) shooting. Usually one finds S&W revolvers so modified.
There should be a revolver builder's name on the barrel or sideplate.
As with any 'custom' gun, value is what someone will pay for it.
I think Dan Wesson is still in production, or possibly under a slightly different name. The revolver's 'big' claim to fame was the interchangeable barrel arrangement.