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Those prices sound really high. I had a 5 ton Carrier complete unit with compressor and air handler with gas heat for $3500,2 years ago. He was a friend but he made money on the deal
Units range from 14 seer to 20 plus seer. Buy a 15 seer with variable speed fan motor in your furnace if you can afford it. The variable speed motor can take out 2-3 times more moisture out of your house making it more comfy in our humid climate.