If your air conditioner is between 10-15 years old, it may be more cost efficient to replace the unit, especially if your unit is no longer under warranty. If your unit is in need of frequent repairs, the repairs may end up costing you more than a new unit in the long run.he average cost to replace an air conditioning unit, without any work to the current ductwork, generally starts around $2,750. This price will include the A/C unit and labor costs. These prices are only indicative of a new cooling system, if you also need to replace your heating system this price will increase.