Mogul light sockets are rated by voltage capacity, usually 4k or 5k, (4000 volts or 5000 volts) and MH's and HPS can use the same socket but each must have it's own type of ballast.
I usually pay around $80 US for a 400 watt HPS re-build kit that comes with everything but the reflector. All I add is the power cord from the wall outlet to the ballast and another one from the ballast to the mogul socket on the reflector.
The lamps are generic HPS bulbs but they do work for someone trying to keep expenses down till they can afford better.
The same goes for MH re-builds.