How much woul it cost for central air with no forced furnace 850-900ftsquared?

You wont know until you have someone come to your house and using Manual J to determine the heat gain for all your conditioned spaces in the house. Not to mention if you don't already have a furnace what it will cost to install the ductwork and the unit itself. Get some estimates because there is no "easy, quick" answer for this one. If you want it done right you need someone to do residential load calculations for your house.