But I don't believe that prefab is particularly green one way or another; I'd call it green-neutral. So don't get taken in by the hype about "pre-fab" houses. There is no free lunch and gimmicks don't work in construction.
The cost of a house is only incidentally based on whether it is site-built or pre-fab. You cannot prefab excavation and utilities or foundations. Appliances and fixtures, carpet and tile are the same no mater the type of construction. (And you can only save on labor to the extent you use use non-unionized "factory" workers.) Most significantly, the cost of finance is the same in either case.
Another subtle but real cost is that the general contractor has to coordinate with all the same subs as if the house was site built. You need every trade on site to button things up professionally. Such scheduling and coordination is not free.
Lots more to say on this subject — just warning that every 5-6 years we see new claims for prefab and then we get abashed silence as the young enthusiasts — hey! I was one — learn the numbers. (And no it is not because of a conspiracy de resistance from contractors etc.)
Btw, where you will get savings is by off-site assembly of components such as roof trusses, which have been adopted as standard in most markets.