Is there a formula for calculating repair cost via square footage of a house?

I also take in consideration the square footage around the community and the rent.

If both units are the same square footage and about the same in other ways, I'd raise the rent to $650 on each unit.

My parents get to stay in a house while I pay their mortgage, I get the deed and an asset for 20% of the market value - "arms length" red flags come to mind here.

Similar square foot, bedroom count, etc. comps in the area are on the water, but the property I am looking at is not.

The details as provided. 10 units(5 Duplexes), 2 bed 1 bath per unit, 640 square feet each, 2/3 acre, built in 1986, new exterior paint, new fence along parking lot, many interior upgrades: resurfaced bathtub surrounds and kitchen counters, tile floors, central heat/air.

@Ashley McMichaelA coop is a share based ownership model based on square footage but I still don’t understand what kind of investment you’re selling them.

BUT look for inconsistencies and red flags with what they say, and what's on the applications.

House purchase price is 240k, 3 bedroom, 2.5 bath, 1520 square feet with a very large deck for outdoor dining, lounging, fire pit, and grilling.  

The costs of a property manager for our vacation rental would keep us far in the red with no real benefit.