Do you have Ducts? Or do you need them installed? Prices start around $4,500 for ducts being there and can go up to $10,000 if not. Lots depends upon location of duct system, Power runs. How far the outside unit is from the inside unit. My suggestion is however try to get at least a 14.5 seer unit or higher. Much better rebate programs so you pretty much get a lot of that extra cost back so price ends up about the same.
I just replaced A/C and Furnace in my 1600 sq ft house. All ducts had to be replaced, cost of just replacing Ducts was $1200.00. Replacing both units 15 seer came to $8900.00. I live in California so rebates and such will be different. But definitely review your state's website for rebate qualifications. They could have limitations on models, sizing, etc. Good luck.