On a CB500X there are 8000 mile service intervals, and the valve check is only done every 16000. No idea on cost from the main dealer though.
I bought the bike with 15.5K kms......now up to 17.6K kms nearly 8 months later.
I'm going to get Juan Cortes to do an oil and filter change at 19K and 22K, then they can do the big one with valves at 25.6K.