Probably the most expensive, because it doesn't exist, is the "Demon Gun" from the video game Killer7. It has a staggered 12-shot cylinder and fires alternately from a top and bottom barrel.
Would probably have to get a gunsmith to build it up from a S&W 500 frame with a custom-made cylinder and two barrels mounted together. The hammer would have to work something like a two-shot derringer's, I guess.
But the action would be pretty smooth, since the trigger wouldn't have to move the cylinder much in firing. Would be heavy, but that would dampen recoil a lot. Reload time would be slow, but this is more of a "fun gun".
I don't have any pictures of it, but it's shown at 0:25, 0:56, and 1:01 on this video.
http://www.youtube.com/watch?v=AdE_mfTQ2ok The cylinder release is styled like Ruger does theirs, which would help because their designs have really good cylinder-frame lockup.
That artist has a really annoying voice.
If we're talking guns that actually exist... um... I really don't know. I like some guns that are hard to get or rare, but none are over 2k. Red9 Mauser, Smith & Wesson 686 SSR, FN Five seveN. A genuine Thompson or MP-5K would be super-expensive, especially suppressed. Or a suppressed Ruger Mk. 3.
Don't we have threads like this pretty often?