TBH Id leave it till it starts blowing unless you would feel better if it was new.its allways going to corrode.nature of the beast im afraid unless you pay for a stainless steel one.
FYI I had a custom made cat back exhaust made for my 2turbo Vectra,7 years ago. Still going strong and looking good and was cheaper than replacing the two rear boxes.
Prices are going to vary depending on where you go,make used and warrenty. I tend to phone round and compare.It is a diy job if you can be bothered.