I'm analyzing a similar project. Actually, this sounds too cheap if you have a three flat and a basement - four stories to renovate (1200 SF- per floors times four?). If you dig out the basement and add new columns, underpin to 9 feet, steel beams, radiant heat, hardwood floors, nice light, sound system, etc. you can be looking at over $150K just there. If four stories, look at fire protection rules. Then, you have three more floors at $120K a pop or so (holding costs $3400/month, 8 to 10 months construction, engineering, architect, interior designer costs, new water service/sewer at $30K)
That is if you delegate everything. GCs need to make a profit. And it all depends what you consider luxury finishes, too. The $550K can become $700K if you want. Then, we have this insane lumber market and everyone is busy.
Any of the contractors doing deconversions will be glad to show you a few projects and give you a quote. I asked my neighbors and they gave me a few contacts. I was quoted $400K with basic finishes for a good old Chicago two flat last year (shakers, white trim, oak floors, two HVAC, electric, water, carpet in basement at 7.5' standard, etc.). I had to provide fixtures. Then, add up your holding costs to that.
We need a recession.