I recently was offered a modern commuter bike with a broken valve. I looked for prices, and valves, piston, head, camshaft, rocker and ancilliary material was over 800 bucks. That's in the same range than original Honda spares, and, what's very important -in my opinion- is that we are lucky Honda still offer the parts. I try to buy NOS when I can afford to, because keeping such a huge inventory costs money, so it has to be reflected on the price. You won't believe this, but it's cheaper and easier in Spain to get original Honda parts than Bultaco or Montesa. It's true that Bultacon and Montesa doesn't exist anymore, but they sold thousands of bikes. On the contrary, Honda never sold officially here in the early seventies, but the parts can be ordered through the dealer and brought here from the headquarters in Belgium in two or three days. Try to get the same parts for 70's Kawas...
Raul