What is the cost to have valve clearance checked? How often should it be done?

The valve clearance on that vehicle should be checked/adjusted every 60,000 miles according to the "normal" driving conditions service table. It is a five hour job, so you are paying the hourly rate x 5. If the shop charges 100/hour, then it will cost 500 plus tax. Shims will probably be necessary as well but are relatively inexpensive.