When comparing bids it is best to do this.
Compare the Carrier bid to another dealer that also sells carrier.
The $6,300 bid sounds like a number for an entire system.
What exactly did the $2500 bid include? That price is closer to a condenser and evaporator replacement.
As far as the SEER ratings... You can get a minimum 13 seer, or as high as a 21 seer depending on the size of your unit.
Please feel free to come back anytime. I am interested in finding out more about your situation.