In general, most attic floor joists are only made to support a minimal storage weight and nothing else; no living space.
There are many factors.
But, to give you a ballpark, it could be anywhere from $50/ SF up to $100/ SF maybe.
If you need any further specific help, please let me know. Thanks,
Not all attics are the same. Some need dormers for more head room. I didn't need to dormer mine, so I saved money there. Some need a staircase. Mine was a one story house, so I had to have a staircase constucted. So to get an accurate quote, the contractor needs to see the attic in question.
Not all contractors are the same. If you already have a real estate broker representing you, ask that agent to give you the names of contractors whose work he's seen.