Yep it’s not cheap, I did this 12 months ago to install a 10kW solar system. Didn’t have to dig pits thankfully so was a little bit cheaper than that.That explains the quote. Someone just quoted me a rough estimate of $4000-$5000 for a Single phase to three phase changeover. Cost includes laying new conduit to the pit, digging , upgrading wiring for the whole house...I have now given up on this.