The reason to do infill housing is not primarily for construction cost reasons, it is to allocate housing where it is needed. Of course it's cheaper to build on undeveloped land but the market doesn't want to live in the middle of nowhere, they want to live in developed areas. That's why you can buy a house very cheaply in West Virginia.Infill affects traffic, schools, hospitals, shopping, energy requirements, sanitation, and water supply. Those effects are magnified by density.
Developers won't be required to study or mitigate those changes. On a per unit basis the savings isn't much, but when measured against infrastructure improvements in already developed areas it's huge. Moving those costs from the developer to the municipality make infill projects very profitable.
This is basic supply and demand and as a conservative you should be glad to see excessive government regulation cut back.