It shouldn't be.
There's a couple bucks of bent steel tube, two $10-20 fittings, and a dozen bucks of bulk hose? Why should that shit cost $300? It's not like there's a professional running the machine that builds it. Half the time it's a highschool dropout. Yeah there's an expensive machine to pay for but a tire mounter and balancer cost more and they charge $10-20 per tire.
The fact that the price is so divorced from the value of the components that's going in reeks of "there is a better way to do this shit and nobody has figured it out yet because nobody has tried"