Just got mine back from the body shop, having had a similar problem. It cost me £400 to get it repaired properly. Think mine was a bit worse than yours. It happens because the arches are double skinned and of a cheap metal. They hold water and rust from the inside. As far as ime aware, you cant buy arches seperatly. Subaru wil only sell you a full rear quater. I was able to source a breakers yard who cut of a good condition arch for me. I then had it welded in and sprayed up. Looks like new now. This may not apply to yours mate, just my experiance of the dreaded tin worm. Rust remedy may work on that. If so, I would of thought no more than a couple of hundred, If that!