Get several estimates and make a decision about what quality you want to get before you buy.
We replaced 2 full units in 2010/11, including all the ductwork (shoddy old ductwork with no insulation). The 15 SEER Carrier units we chose were about $1000-1200 more expensive than the York brand the installer also used. We went with the Carriers, b/c of their excellent reputation.
Our electric bills dove into the basement. We live in a subdivision with a very high RECC provider (about 40% more than the provider for the rest of the county - wish we knew that before we bought this house). Our electric bills are consistently lower than everyone else in the subdivision, by a good margin. I actually set the heat higher and the A/C lower than we did with the original (7 yo) units, and our bills are consistently $100-150 lower than they were before. This is a combination of additional insulation, the new ducts, the new units, and CFL/LED bulbs, but it is a real savings that will pay for the replacement cost of the units in about 8 yrs. And b/c they are Carrier, I expect they will last 15-20 yrs.
Oh, the cost: basement / 1st floor unit including new ductwork, adding additional ducts for finished basement, and auxilliary outside air return $8900, attic unit for 2nd floor, including new ductwork $6700. The math for 8 yrs factors in a 30% rate increase that is not included in my monthly savings based on original bills.