1 For an 1,800 square-foot house, a new 13-SEER unit Lennox air conditioner will cost around $3,000 to $4,000, while a new 16-SEER unit will cost you about $5,000 to $6,000 for super-efficient units.2  For instance, the Lennox Merit series price ranges from $1,200 to $5,700. Centralairconditionerprice.com notes that the average air conditioner and coil installed cost about $3,420 on average. 2  Users on FatWallet.com noted that they paid anywhere from $5,200 to $9,000 for their Lennox unit to be installed.