As far as the piston and bore, mine was ready for a refresh of the piston and rings at about 4000 miles (bore was still in spec, just did rings and piston), and a new tbig bore kit at about 7K (the cylinder was worn out of limits at the bottom). The big end bearing and mains still felt good, though I replaced the crank at 7k as well because the splines on the primary drive
side were worn. A good rule of thumb is if the valves "move" out of limits twice in a short period of time, it's time to replace them and grind the seats. As far as cost, it was about 1600 bucks for everything...new crank, new big bore kit, 5 new valves with springs, retainers and keepers, grinding the valve seats and cleaning up the ports, gaskets, etc.