Keep in mind the NM 44 50th (50th Anniversary) Flattop is a hybrid--if referring to that. It's really a large cylinder frame with a midframe gripframe--overall effect stil being the larger-framed gun. To get a "full" flattop effect and keeping the size to a midframe, I would say perhaps converting a 50th Anniversary .357 or 44 Sp Flattop by replacing the barrel and reaming the cylinder--I'm assuming cartridge OALs are not an issue, or better yet swapping in a New Vaquero cylinder--but someone more knowledgeable can chime in on that. I also don't know if cylinder-barrel gap needs to be "adjusted" in such a conversion. A Bowen, Clements (etc) or other qualified smith's installation of S&W (or other custom) rear sights and new "adjustable" dovetailed front on a New Vaquero .45 would functionally accomplish similar objectives, but obviously wouldn't give the look of a flattop if that's also wanted.
None of the above really answers your cost question. If you can find the "right" (benevolent!) smith, I'm assuming new sights worked into the New Vaquero could be the cheaper route unless a donor NV .45 is at hand in combo with a the 50th .357 or 44 Sp Flattop..