That's the guide I've been following.
When mine were checked by the dealer at 33,000 miles it was c. £100 But I was told it would be a lot more expensive if they needed adjusting. From doing this myself, I would actually say there's not that much more to do to swap the shims as most of the time is spent stripping the bike to get to a position where you can check the gaps. From checking the clearance, you only need to remove the cam-chain tensioner, cam journals, buckets, shims, reassemble and recheck the clearances.
Removing and refitting the fairing, tank, airbox, radiator and spark plugs are all part of checking the clearances anyway.
The shims aren't that expensive either. I originally bought some from Wemoto c. £20 for 4 (2× two sizes), but have just ordered some hotcams shims from an MX website c. £20 for 10 (5× two sizes).