Sounds like a tear down and build new would be the most cost effective.

Your request is impossible to respond to without any pictures or firsthand knowledge of the structure.

Call some local contractors and have them provide an estimate.

I believe that new construction can range from ~$75 to $125/sq ft. That might be a start.

That's basically everything that makes a house except for the foundation, studs, and insulation (which should also be addressed). 85-90% of all of the most expensive stuff at least. But, renovations cost more than new construction per square foot because the walls aren't open, and you're trying not to do damage to stuff that will be saved, and it's just plain harder!

So, a teardown does make more sense. Especially in a high land value location like Boston.

In a lot of the metro areas there, the red tape to do a tear down May make it easier to just do a complete gut to the studs and rebuild rather than an actual tear down. Sorta a tear down within the existing footprint. It won't be cheap, but blowing your nose there probably costs you a $100 permit and environmental impact study.