L.M.
ETA: ours are 3 ton units, and it took almost 10 hours to complete the job.
We just had 2 units replaced--our upstairs and downstairs. We had both the outside units and attic units replaced. They are top of the line, 16 SEER, and our total cost for installation & parts was around $15,000, and that is with a discount bc my hubs uncle got us the parts at cost. Yes, my husband pointed out that is as much as a car ;) There are so many variables as to how much this could cost: what parts are needed to be purchased, the SEER rating, outside only units, inside only, both? Etc...I suggest doing a TON of research on this before buying ANYTHING. Don't just call around & ask prices. Look at all of your options first, and get what suits your needs for your house and your climate. It may not be as fun as buying a new car, but it's still very worth it! :)