Pistons of the same size from the same brand, here in NZ at least, are the same price. They should have the same RRP from the manufacturer too.
However, dealers will sell more of a jap bike piston kit than a gasgas one, so they sell jap pistons for less, but sell more of them. Just economics.
Also depends on whether they are getting price breaks on buying in bulk, so a dealer can probably buy 200x YZ pistons at a 20% off wholesale, and sell them at a 15% discount off RRP, without too many problems.