My future husband and I just bought our first house. The house is a small cape cod-936 square feet have already forced air, a new furnace and all the ductwork is already in place. With this in mind, what would it cost to add central air. What I discovered was – A two-ton central air conditioner is all you need for the household size. It would help if we knew where you live. Prices vary by region, city, etc. And brand of the unit can make a big difference. In the Midwest you can have a 2 ton unit installed between $ 2100 – $ 3200 for a 13 SEER model. The price includes the reel, line-set, the outdoor unit electrical disconnect and pad. Make sure you have the supplies return air installed on the second floor or not you go to the proper air circulation and upstainrs be much warmer than down.