In my area, the independent certified installers are significantly more expensive for the same thing. I was calling around yesterday as I decided to get serious about powerwall(s). I called 2 certified independent installers in my area, and both told me the SGIP was fully subscribed for my utility (SCE) this year (negating that benefit) and their costs were "13.5k for one powerwall, 20k for 2, including permits and basic installation" (the other certified installer was 21.5k for 2 powerwalls)
Note, they both had around this price, and on one of their websites it specifically calls out that "they can not do it for the price tesla charges as that leaves no profit for the installer". This pricing was before they even specced out my install which admittedly should be very simple, as the powerwalls should be able to be placed within 20 feet or so of the panel, on the inside of my garage.
Both basically intimated that the pricing above was sort of the "basic" all in price, and it wouldnt get cheaper than that, only more expensive.
I put in a request for info on tesla's websit yesterday evening, Tesla called me back very quickly, (today, at lunch time), and told me they are doing installs in my area. They quoted me the basic prices on their website which dont include install. 6,500 per powerwall, 1100 for the gateway, they quoted me 500 for a permit, and then told me the variable was installation costs, but expected those to be somewhere between 1-2k depending on my setup, if not extensive. So, somewhere between 15.7 and 16.7, give or take, where the certified installers were 20-21.5k for the same basic quote.
So, significant in my case. Will let you know how it goes.