What is the approximate cost of an alternator for 88 Chevy Nova that shop had to go across town get?

There's no way to give an exact cost without knowing the specific alternator needed (different engines have different alternators) and the shop's markup. However, a replacement alternator for a 1988 Chevy Nova could range from $50 to $250 depending on whether it's a rebuilt unit, a remanufactured unit, or a brand new one from an auto parts store or online retailer. The fact that the shop had to go across town doesn't necessarily add to the cost of the *part* itself, only potentially to their labor charges.