Sure, the profit margin would have to be wide enough to account for a lot of variance.
Seriously, the insulation effects on your heating bills cannot be underestimated. Two houses that look superficially the same on the curb can have a margin of error that is a multiple of your heating bill, and that largely comes down to what the insulation is in the house.
Like we're not talking about "variances up to 30%" territory, we're talking "variances up to 300%" territory. No one is willing to make that guarantee without pulling off your drywall and examining the state of your insulation, not to mention the various behavioral issues others have brought up that vastly alter your bills.
Seriously, the insulation effects on your heating bills cannot be underestimated. Two houses that look superficially the same on the curb can have a margin of error that is a multiple of your heating bill, and that largely comes down to what the insulation is in the house.
Like we're not talking about "variances up to 30%" territory, we're talking "variances up to 300%" territory. No one is willing to make that guarantee without pulling off your drywall and examining the state of your insulation, not to mention the various behavioral issues others have brought up that vastly alter your bills.