Help!

Our central AC unit in our house needs replacing. I've seen estimates online ranging from $5,000 - $13,000. We have had 2 different companies give us estimates, and both companies said $24,000! Doesn't this sound unreasonable?! Does anyone have any knowledge on the subject, or any advice? Even if I could easily afford the $24,000, I wouldn't pay that much. By the way, our house is 2400 sq.ft. and 2 story if that makes a difference.