I've had one done on my borg. Was $950 for a 3" mandrel straight through no cat, multiple flanges and a cannon. That was just mild steel, excellent workmanship. I will get the same place in Perth to do my rvr's soonish but will include a cat so i am expecting it to be about $1150 all up.
What size exhaust are you interested in getting? I'd expect to pay about 1k to 1.5k for a custom made exhaust with all the bits. Muffler around 300, cat around 250. The rest in pipes (3"), flanges, gaskets and labor.
Keep in mind, A full system can be great, but if the shop doesnt understand the limited space coming off the turbo, then you're going to be in for an interesting result...