Most people in business have a "rule of thumb", eg in Fish and Chips there is 1/3 Materials 1/3 Overheads and 1/3 Profits. Every portion of fish wasted, every extra scoopful of chips, hits profits directly.
What do developers look for?
Round here (Royal Borough of Kingston-on-Thames) every 16th flat you build has to be given to the council, the result is that developers are shunning big plots and squeezing 14 two-bedroomed flats into the largish gardens of two demolished adjacent detached houses.
Anyone know what sort of return they look for? How much each flat can be built for? How much the plot will be worth? How much profit they expect and what they base their expectations on?