Most expensive price wise would have to be the S&W model 629 I bought, it had a really neat looking custom grip (some odd orange wood) and a really unusual trigger. It was used, barely, and a friend's dad bought it new at some gun dealer where he lived for a while. It had the name of the gunsmith who did the work on it. I paid $600 for it, shot it once with a friend, who instantly offered me 850 for it, and I took it. He still has it, and I still regret selling it.
Most expensive time wise would have to be the horrible little FTL Auto Nine, or "Auto One" as I called it after it blew the extractor on the first shot. There's one on one of the auction sites right now. Between the trips back and forth to the manufacturer and the high price it cost back then, it was a real dud. The Beretta 950 Jetfire I replaced it with was about the same price and will soon be 27 years old, and has had zero problems.