How big of a tax cut would Apple need to sell its products here in the U.S.?

it already sells it's products in the U.S.. You are thinking about what kind of tax breaks would they need to make everything here in the US. They don't.

The reason they make their product in other countries is purely profit driven. If they made them here that would translate into less profit for them, being in the business to make money they go where labor and parts are cheap and then sell it at a ridiculous mark up to people who are willing to spend insane amounts.