It's hard to say unless we can know the extent of the rust and how you want it fixed.
If you want to fix the quarter panel the cheap way, a body shop can probably bolt on a metal plate with mounting points that the bumper can attach to. It won't be pretty but it will do the job. Cost probably about $250-$500
If you want it done the proper way, then they have to see the extent of the rust. They will have to cout out the rust, weld in comparable sheet metal, patch up the body in that area, sand it, and repaint it.
If they repaint it then the shade of color might not match the rest of the car because paint fades over time.
So a professional job to fix it and paint it would probably run you $500-$1000 or more depending on the extent of the body work and rust.