The cost really depends on your location. Costs vary everywhere.......

I suggest that you talk to three local architects and ask them to bring in a contractor so that the discussion of costs is between you, the architect and a contractor.

You should get three different opinions and then decide.

Select one of the architects to work with and go for it.

Even though I don't know what the costs will be where you are located, it won't be much less then new construction because the only thing that you will be re-using is the concrete slab. Everything else will be practically new construction......

and please don't get discouraged. It is wonderful to restore and re-use old buildings like barns. They have so much more character then new construction and you are being very sustainable.....