@Josh Parra is right about the costs. We have converted multiple 3-families into condos and those numbers are spot on.
We've spoken to others who work in higher end markets (South End, Beacon Hill, Brookline, etc.) and the cost per sq ft can be much higher.
@Ann Bellamy brings up good points as well about dealing with the city. We are working on a new construction project in South Boston which included demolishing a house and it's probably 3-5x longer waiting for approvals, meeting with city officials, getting utility/site work performed, etc than it is for the actual building of the house. Getting to know the inspectors, never cutting corners, and hiring competent workers with relevant experience is the only way to go, otherwise it will cost you more in time and money.
@Rick Jones If you have the funding to do one of these projects, go for it, but the competition is stiff and in an area where values are mature and high, it will be tough to get a "below market" deal. It can be done with direct mail, but it will take time.
I also feel your estimated numbers with regards to the down payment and rehab are too low - it's really crazy how expensive it is to work in the city. 150-200k might be enough to rehab one unit if you're in the dense/difficult areas and are putting in high end finishes to get top dollar. Consider other markets near the city to gain some experience at lower rates - there are many up and coming areas or areas that are in the middle of a turn. We have worked in Dorchester and it's a good balance of dealing with the dense city issues, but also at a reasonable price point (relative to Boston, still way above the national averages).
Hard money on a $1M+ deal... yikes, good luck. If the hard money lender approves it, you either are putting 15%+ down or you really did get a good deal. Run the numbers and just imagine how much more you could get with a commercial loan. Or consider working with another experienced developer/investor to bring a deal to the table and some cash for a possible joint venture! It would reduce the risk of unforeseen issue due to lack of local experience but also get you ramped up learning the process much faster. Your profits might be the same as working with a hard money lender, but the lender isn't going to help you with the project.
What kind of rehabs have you done before? The details in your profile were high level. Also, do you market in-house or out-source? We tried both and prefer the latter.