Obviously completing a frame off restoration will have a better result than simply replacing the rear frame rails.
Unfortunately not everyone has the time, space or money to do so. Once a body is off the frame there is much more to the process than just "swap frames". "Everything" under the body will need attention. including the under body.
I've sold several pairs of used frame rails in the past few weeks. It is a fairly simple job.
Even as simple as it is you'll need new body bushings and bolts.
I'm sure once the fuel tank is removed, there is that pesky old sending unit and hoses that you'll want to change, maybe even the fuel tank and clean up those straps. May as well clean up and paint/undercoat the trunk area before the tank is replaced.
Now think of all the upgrades that should be addressed once the entire frame is removed.
These are two totally different projects. Similar to comparing patching and painting a rusted fender and stripping the car to bare metal and doing a full repaint.
So as far as cost, I'd be guessing if I tried to quote a welder in your area. You will save a lot of money if you could remove the fuel tank before you tow your car to the weling shop.