[wardlow]

There are costs of infill, but in some cases those costs are:
1) much less in some specific areas than in others. There are some areas that are, on the surface, prime candidates for more infill have major service capacity issues, so costly upgrades are needed, whereas a few blocks away there is existing infrastructure that is under capacity;
2) costs that won't go away by banning infill and pushing all growth to greenfield areas. Existing infrastructure (eg, combined sewer systems) will still need to be replaced regardless of if you increase growth or not.