Was wondering if anyone can give me some advice about rust repair inside both the rear wheel arches of a Type 3.

Is it a case of getting sheet metal bent and welded into place? Or would you need to get a replacement panel for instance?
Also, can anyone give me a rough idea of the cost? I know it's a bit tricky as you never know what other surprises will pop up, but are we talking $100's or $1000's here?
I've had a good look around and the rest of the car and it's all pretty clean. It's mostly just these bloody arches giving me a problem now!
Apprecaite the help.
Cheers