Remember, just because your increasing the volume of you heated space does not mean that your increasing the potential for heat loss.
For example, a 24 X 40 house on a crawlspace. If you insulate the floor, you have 960sf of area from which heat can conduct out. If you insulate the perimeter 4' down, you have 512sf. Much of this would be blow grade, so the exposed to heatloss is lessed the the insulating quality of the ground.
What of the crawlspace floor? 2 factors to consider: 1/ remember the ground is an insulater. Granted it's a poor one, but it's also thick one. 2/ The exposer to heatloss is less because the temperature difference is less. Instead of the outside air temperature, you looking at the ambient ground temperature. The net effect is less potential for heat loss although having greater interior volume.
As far as the cost difference in installing, there is little difference, on new construction or retrofit. As a contractor, I would charge less for doing it as I've layed out.