How much or how do I charge for the post cleaning of a new construction by the job or by the foot and how much per ft

You make a list of every possible cost to you, from dumpsters to cleaners to labor to WC if you hire anyone. You go look at the job and take notes and ask questions. You THINK. You go home and imagine each room. You add some hours. Then, only then, do you calculate by the square foot, because you clearly have never done this before. Prices will vary, and you have to teach yourself how to estimate and what to ask.
Remember - a builder is asking you in the hope that you will underestimate, which most newbies do.