Does anyone know the cost (estimate) to install central air conditioning in an older two story house?

I have looked at a number of older (1920 houses) and some of them do not have central air conditioning. This is not in a cooler part of the US either. Having lived in these areas in the past, I know that window units do not work well. Do you have any idea what it would cost to install central A/C in a house? Ball-park? If it has forced hot air vents already? If it doesn’t? Is it a very difficult process? I just want to know what to figure into the cost of the house.

Any info would be appreciated.