Older duplex needs central air

[Keelia Purscell]

Hey there fam, 

I'm finding quite a few older 1930-1960's duplexes and fourplexes in my general area here in DFW that don't have central heat/air.  They are currently rented with window units/space heaters.  This would seem to me to be the first upgrade that is an obvious fix here in central Texas.

What's a ball part figure for a new system? Does it run price per square foot? Is each unit a separate bid?

Thanks all