They're about the same when complete.

The land costs the same. The well and septic costs the same. The foundation costs the same. The driveway costs the same. The landscaping costs the same. Running the power costs the same. The HVAC system costs the same.

So when all is said and done you have a comparably priced home that is worth just the same as a site built home.

I have a modular. I like it very much, but the only savings was time. Here in Vermont we have a short building season, so modulars are pretty common. We ordered our home on labor Day weekend, and had Thanksgiving dinner in our new dining room.