Honestly between equipment costs and labor (shortage of skilled tradesmen) both going up, 2x is pretty close to what I've seen in other areas of the building/construction industry.

In my view Mass Save is only contributing indirectly by increasing demand. I doubt installers are just adding the rebate since competition should keep that in check. Otherwise you're talking systemic market failure or a cartel among installers. The latter seems extremely unlikely, though the former may be possible. I honestly don't know if there are barriers to competition preventing new firms from starting/moving into state, or existing firms expanding.