On my 04 636, the manual said at 18,000 miles check the valves. The dealership quoted me $250 labor plus parts (shims, gaskets,etc.). Since I'm a big DIY, I did mine a few months ago. It was actually quite easy, but time consuming.
I had 8 valves that needed adjusting. The head mechanic at the dealership helped me swap out the shims I had for the ones I needed for each valve. I put new plugs in while I was there too. I believe it cost me a grand total of $25 and a little bit of my time.
I'm actually going to do my buddies 05 ZX10R in a few weeks. He's going to donate $$ to my new tire fund.