For a VW dealership doing the standard dealership work, that's actually a pretty good price. They don't just replace the CATs, they replace the entire exhaust segment that contains them. You can purchase and have installed direct-fit (don't try the "universal fit" ones) CATs for much less.
HOWEVER, you might want to look into exactly WHY they think your CATs need replacement. In some cases, a bad CAT diagnosis on the V6 can be an over-sensitive monitoring program, fixable with a firmware flash.