How much would the expected savings be on a 2500 sqft ranch, new construction in West Mass, if we decided to build on a slab versus a basement. Plot is a top of a steep hill but flat on the building site?? Any referrals about builders willing to undertake project would be greatly appreciated.

You should check with what is the norm for building in your area. Someday the house will be sold and if the area is a basement area like we have in NY then it could hurt the value.

The savings should be greater if you use a frost protected shallow foundation slab.

Thank you for the advice!

In the desert south west frost protected shallow foundations are a matter of everyday fact. So it is in areas of permafrost. I wouldn't build anything else.

http://www.advantageicf.com/media/cold_ground.html

Worth reading, although I disagree with that reference in terms of the heat coming from the building. IMO, the best FPSF designs use natural ground heat, not heat from the building. So they also work for garages and unoccupied buildings.

Indeed.

You won't know until you dig a hole. It's a gamble and you are at the mercy of the dirt you are building on. What kind and how deep. If you can mine granular fill on site that is a huge bonus. A local excavation contractor would be a good person to talk to.