I paid about $6400 5 years ago for a mid-range, 5-ton Trane unit, installed.
Given that this was 5 years ago, we were in the midst of a recession that really wiped out HVAC companies around here, and I live in the Phoenix area where labor is cheap anyways, I'm not surprised that many of your quotes are higher than this.
If you want to save money, make sure the contractor does what is called a 'Manual J Load Calculation' to actually size the unit correctly. Many contractors will just size the new unit the same as the old one...without taking into account whether or not the old one was sized correctly to start with or if you have done any improvements/additions to the home that would increase/decrease the need for heating and cooling. If the contractor doesn't know what this calculation is, or refuses to do it because they've "seen a bunch of houses this size in this area and already know the right answer," tell them 'no thanks' and call a real professional.