The assets in the rate base are depreciated, so fair market value could be higher than that. Also we don't know what is and isnt included in rate base asset. Does it include their office buildings? Trucks? I honestly have no idea, but I'd imagine it's less than the total FMV of CMPs assets. The maine utility regulator might have a more detailed breakdown.

That said I doubt there's anything stopping CMP from inflating the number to 13.5bn in the commercial. So your answer is really it'll cost likely more than 2.7bn, but less than 13.5bn.