Why there is no per-minute or per sq ft rate
A film has a length, so it can be charged per minute. A real-time walkthrough has no length. One visitor spends a short while in it and another explores every room. Nothing is rendered in advance. The picture is drawn live as the visitor moves.
Area does not work as a unit either. A small flat where the visitor can switch the flooring, the kitchen and the furniture is more work than a large flat with no options. So an Unreal Engine walkthrough price is quoted for the project, against a written scope. If a supplier does give a per sq ft rate, ask which options and platforms it includes.

