Here is a link to a website which offer's custom exhaust manufacturer's parts and products particular to their trade, as well as a list of Custom Exhaust Manufacturer's.......
http://www.coneeng.com/local_fabricator.html
And this is one of the manufacturer's that caught my eye, check out their gallery under "Exotic's" they have done Lamborghini's and Lotus' btw, their price for all SS downpipe's including test pipes on an RS6 (which consists of two units) $1200.........interesting to say the least, especially for such a small company. Mass production should definitely reduce cost......
http://www.labreemotorsports.com/gallery_exotic.html