Electric or gas?
Rough guesses on sizes for Georgia climate (100,000 btu gas furnace, 5 ton ac), figuring it's a basement. Ball park is 8,600 to 12,160 installed, depending on equipment, access, and the contractor's markup practices. This is just my opinion, but somebody charging under $8k for a remove/replace is either not providing himself a living wage or staying in business, or he's selling less than good stuff. You might get it that low if someone is hungry for work, but understand that something that low carries risks.
You'll probably get a couple hundred bucks in a rebate for a high efficiency unit, and your contractor might also qualify for a rebate. Ask him about both.