Rx7 calipers may be unique to that platform. That would explain the price. 300zx and vr4 may have some slight difference to give it a different part #. I don't know much about those applications so I can't be sure.
I will say that resource isn't perfect. For example, my 91 toyota pickup and widebody SQs use the same front brake pads. If you look up the pads for the toyota the compatibility tab says they only fit the toyota. If you look up the SQ pad it says those pads fit dozens of vehicles including the toyota.