$4,200 is _very_ optimistic for the panel/battery cost for a 30kWh system. The GP comment also conveniently excluded "labor and extra hardware/electronics". There's a lot more that goes into a solar power system (especially one that has to manage batteries) than just slapping the panels on the roof and dropping the batteries off in the garage. Inverters, chargers, grid integration equipment, etc.

I'm also not sure what you mean by "just" labor. You're talking about a critical system, that has the possibility of seriously injuring someone (including folks outside your house) or causing extensive property damage if not installed correctly. That seems like a thing where skilled (and fairly compensated) installers and some degree of regulation is very appropriate.