Cost of copper
Petey, if you presume prices will continue to rise, you'd be wise to buy as much as you can safely afford now, and charge 'current market price' when you do each job. Much as I hate it when folks (selling to me) do this to me, it IS the right (from the capitolist pov) way to handle it. Of course, if EVERYONE does the same thing, it drives the prices up higher. The flip side of that, is that if everyone except you does the same thing, the 'current' prices still go up for everyone, but YOU end up paying more than the other guys that bought early.
It is only when prices are falling rapidly that you want to have 'minimum onhand inventory'.