I've seen a few places do it for 350 before but also seen it done for well over 500 just depends on who's doing it. Woukd say that's an OK price.
FYI Rtech have been doing a deal for a while now where they will do a carbon clean, injector cleaning and a tune for 650, bargain price really. Think AwesomeGTI are one of the ones I've heard doing just a carbon clean for 350 from memory? If either of those places are near-ish you that is.