A rebuilt caliper is a rebuilt caliper. If I am not desperate for the part and they aren't the only ones that have it, I will go for a cheaper price. 5 or 10 bucks and I'll buy wherever I find one first, but when it's 40 bucks more, no sale.
I don't buy from NAPA. And, I don't really care if anyone buys NAPA or not. I am just explaining why there is a difference in price.
There is however, potentially a big difference in rebuilt parts. Not everyone has the same standards, and tolerances. A lot of rebuilders just put in what has failed, the rest gets put back in service, to fail later. Or, they use low quality Chinese bearings, O-rings, and seals, that have a fraction of the service life the OEM parts have.
It's not at all uncommon to see brand new budget Chinese Calipers, and wheel cylinders, which have all but replaced rebuilt, come out of the box without any threads cut in them. So you box it up, and go back, and find out, that was the only one they had, (of course).
After decades in the automotive business, I know from experience, a cheaper part is
usually not as good. On the other hand, sometimes on an older vehicle, cheaper is fine, they will still outlast the vehicle.