Schomp walnut blasted the intake valves on my 2008 335i under warranty at 45,000 miles, so not sure what they normally charge. My service adviser told me it was the first one they had done, which surprised me. I can confirm that it made a huge improvement in power delivery at highway speeds. Smooth and strong, just like new. I suspect this will become routine maintenance on all cars with direct injection, so I'd be curious to know what the indie shops charge. Good luck with your search.
2008 E90 335i >Blue Monaco, Lemon/Aluminum, 6MT, Sport, Prem, Cold, iPod, CA, Sirius >European Delivery in April 2008