The most obvious cost of detached homes is the completely unsustainable amounts of infrastructure required to maintain them. Roads, sewage, electric, etc.

It’s a well documented fact that suburbs of sprawling suburban homes are bankrupting towns/cities all across America and only the densely built downtown cores are keeping these cities afloat because the tax revenue of dense mixed-use areas is substantially higher than the cost of maintaining the infrastructure for these places. Check out Strong Towns if you’d like to know more and see the studies showing all this.