For new construction, I recommend reading a book called Building an Affordable House by Fernando Pagés Ruiz. Awesome book with lots of tips to build a very cost efficient, high value house or multi-family property. $100/sf on the construction side is fairly accurate, but there are a lot of unknowns that need to be figured out before you ever start construction that will significantly impact cost. Estimating things like cabinets and countertops, flooring, roofing, etc are really easy to determine the $/Sf, but the real killers are stuff like bringing underground or overhead utilities into the property, the type of foundation and how much excavation is required, and making sure your units have a very efficient layout for framing to minimize material waste and the degree of difficulty for the carpenters and other tradesmen.
