I think the way the prices of these new are climbing suggests that the stock of them is running out, or at least getting low.
The RS4 was a low volume car, so it would follow that audi wont have made tens of thousands of them, they are popular in the aftermarket though, so theres a far greater demand for them than the actual number of cars produced would indicate. I seem to recall manufacturers are required to supply parts for any vechile they produce until 10 years after they stop production, so assuming their supplies hold out until then (2011?), you can bet audi wont be manufacturing any more, and they'll just keep nudging the price up to try to keep the demand in check.