I am about to bid on a house that has forced hot air and central air throughout. When I saw the house, I noticed there was a wall unit ac in the upstairs window so I asked why it is there if the house has central air. The explanation I was given was that the house has all the ducts and the air does come up to the second floor but there is no "return" from the second floor.

What is "return"? How would I get it installed? How much would it generally cost? Any information is greatly appreciated so I can determine my bid.

To install a return duct will cost some $$. I suggest you contact a local HVAC company and get estimates. Return is just what it says. It returns the warm air back to the unit to be cooled. This is a common problems especially with older homes.

no 2nd floor return = no cooling on the 2nd floor