My Fusion was bought as a pre fitted slide, frame and barrel. All other parts were fitted by me. Without including my time over 6 weeks. The pistol and tools put me over $1300.
It can depend on your goals and the standards for the build. I wanted mine to be a tack driving target gun. So, I took my time and bought high quality parts. You can buy cheaper frames , slides, barrels and other parts, and if you aren't looking to maintain the tightest fit with smooth function, they can be cobbled together for less, and still give you a reliable shooter.
Others already pointed out that the frame still has to be purchased through a licensed dealer. The frame is the firearm, to the ATF. It is also the part you want to be the most cautious with when fitting parts. It's often less expense and less hassle to fit the smaller and cheaper parts to the frame, and if you do mess up you only have to buy the cheaper part and start over.
Remember these guns were designed when labor was affoardable and CNC didn't exist. Most "drop-in" parts either don't just fit, or they have a lot of slop or big gaps and look kind of cheap to any other "1911-person".
I looked at building my 1911 as a commitment to getting it right with little threshold for cutting corners. I left the big and expensive parts fitting to the experts. It all worked out well for me. I have a precisely fitted 1911 that functions flawlessly and shoots tight groups.