What size? If you are wondering the actual cost of furnace alone if you were to be able to buy it (which you can not, Hunton Trane is very strict as to whom they sell too and Trane owns American Standard, virtually one and the same) versus purchase and install cost through an hvac contractor I can tell you right now that install trumps everything, name brand and whatnot, and can easily cost three to five times the cost of just the furnace alone and that is simply how it is in the trade. And geographical location also has predetermined prices. What you may pay down here might cost you a few thousand more up north or vice versa.
You have plenum and possible duct modifications if not new, transition modifications if not new, new flue pipe, new heater feet, new flex gas and possible rerouting of existing gas line, new return (duct) plenum or modification, new gas cock, a lot of other work than just the price of the furnace.