Is it possible to upgrade a MacBook processor?
Since the CPU and GPU are welded onto the logic board (Apple's term for motherboard) such a task would require you to replace the whole logic board!


I cannot name specific prices, but this is really expensive. After all, this is the most expensive part of a Macbook. Usually, it is recommended to buy a newer (second hand) computer instead.
An upgrade to the logicboard of a non-unibody Macbook with a Core 2 Duo might be possible with e.g. the generation of the Macbook 4,1 since both are build quite similar. You should verify this with the Apple support.
An upgrade to the logicboard of a unibody Macbook with i3/i7 is ruled out since the internal structure is totally different. The size and location of logic board, and battery have changed 'vastly' amongst other things.