Mostly the per sq.ft price comes with new construction. After the home is built, it becomes somewhat obsolete as some have finished basements, some do not. And you cannot have a price per sq.ft. when it's much less expensive to finish a basement that already has exterior walls.
I believe what you should be looking for is the comparable pricing for the type of property you are/will be looking at. In the end, it'll be what type, price range, style, etc. that speaks to you.
If you need assistance I would be more than happy to discuss this with you.
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