Yes, TH Lawrence, it should be ‘square foot’ in that context since the reference is to only one thing – ‘one square foot’. When you say ‘per square foot’, that is basically a meaning similar to each (one) – i.e. singular. So, in the first A sentence, each square foot costs $100.
If you refer to 500 of those things, for example, then you’d be referring to more than one square foot, so it would be ‘500 square feet’. The abbreviation ‘sft’ in the second Q sentence means ‘square feet’ because it refers to ‘1000 square feet’.
You would also use ‘foot’ if the square footage is used as a hyphenated adjective that modifies a noun thus:
This building is too small for your business. What you need is a 40000-square-foot showroom.
Here are a couple of quotes illustrating ‘square foot’ and ‘square feet’: