For the price thing, one word. Volume.
They sell thousands of oil pressure gauges for every boost gauge. The can charge what they want.
Sucks, but that's it, I think.
There should be no issues with the long run. Pressure in one part of a closed system, is going to be the pressure in all other parts of the system. Might change up some of the tuning, if you were using massive hose, but the 1/8 line that is used is pretty much invisible to the engine.
You could always scrounge around the wrecking yards for an electric one, but ... how much time is that gonna take?
Cheers
Trev