I had Greg Derr build me a 1911 Caspian Commander-stainless. Very happy with Greg A++
Based on my experience: yes it can run you $2K+, but you can also probably do it for $1K or maybe even less as is mentioned. Like anything alot depends on what you really want.
Personally I chose several options for my slide & frame from Caspian that made these components an expensinve starting point like some engraving, custom serial #, checkering, magwell, etc-for me the gun is something I will keep forever and my sons will probably fight over some day
So if you want to personalize it or trick-it-out a 1911 is just like a car or anything else: you want a stroker kit, dual-quads and a bitchin
set of mags -it is something you can spend alot on.
At the same time, whether it is through the group buy or specials Caspian offers you can get a slide and frame for $500 +/- in some cases. It may have been mentioned but Caspian has a sister company they link to off their web site named Foster Industries where they sell frame/slide overruns and blems for even less in some cases. Other resources have been mentioned here-Point here being you can get a slide/frame cheaper and have a gunsmith build you a great/accurate pistol without a $2-3K investment. If you are comfortable doing your own fitting/smithing you can save even more.
As far as putting a 9mm top end on your .45 this is also another option I would talk to a gunsmith about. You could also buy multiple slides and talk to the smith about building top ends in .45, 9mm and .40 for your frame.
I would advise that if you are going to use a gunsmith like Greg Derr or any of the others recommended on this site, you talk to them first, they can give you input on the options available-particularly any machining to the frame and slide. IMO one of the biggest factors in cost can be deciding if you have a specific use in mind such as competition shooting, carry or just for fun.