GP's argument is the marginal cost when building new is roughly that amount, not that any house can be retrofitted for that amount.

However, it's not that far off for retrofitting, if you do it when your siding already needs to be replaced. Add 3-5" XPS foam to the exterior of any standard house; if a basement you bring insulation several feet down and out below the ground. If cathedral ceiling, when replacing the roof you put 6-8" polyiso down over the sheathing before the new roofing material. If vented roof, get 1.5x code minimum blown in the attic. Air seal first, of course (1-hour of air sealing is the best ROI of anything you can do in an old house).

But nobody wants to put that money up.