Anglophile

A: What is the average cost of construction nowadays?
B: It is Rs.1,500/- per sqaure feet.
A: So, a house of an area of 1,000 sft costs Rs.15 lakh, doesn't it?
B: Yes, of course.
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Now, here, 'per square feet' means 'for each square feet'.
Each is singular, meaning one, in which case feet does not agree with per or each or one.
Now, mathematically, area means length multiplied by breadth. If length is 1 foot and breadth also is 1 foot, the area is 1 x 1 = 1 (one).

We may, therefore, say 'per square foot' or 'for one square foot'.