If you're willing to go with LLTek, they have an RS6 spoiler for $333 (
click here). But, of course, if you get paint match, you're looking at $444. Also, the LLTek spoiler is probably aftermarket, not sure if the material is cheaper though.
I went with the bigger spoiler they offer and love it (
click here). It was also cheaper than the RS6 spoiler ($288). Not sure if it's you're style, but I thought I'd throw it out there anyway.
LLTek also offers an aftermarket rear valence for the A6/S6 that looks like an RS6 (not on their website yet, only in their catalog). It also costs $333 before paint match. If that's an option for you, it will probably be cheaper than getting a full RS6 rear bumper. LLTek says it's an easy bolt-on application, but I haven't found anyone who has tried it yet. Although they do mention possible trouble if you have low-mounted backup sensors in your rear valence.
Damn! I sound like a fraking LLTek salesman...