The cost of materials and how many walls per square foot. Then if there is a huge difference between lots it's the price of furniture added. But material used in each apartment is just as important (doors, wall coverings, windows, siding, tiles etc.) that all adds up to. It's cheaper when you build an apartment building with that $2 paint etc. (or $0) You can test while building by not adding anything to see which unit starts out higher than another. Walls cost money, too. ETA: Also the size of lot you began with also is divided up.
