Duckman.......well said.................GeorgeI think what George2 is trying to say is in most all cases a (3) ton would be causing you problems with short cycling, increased electic usage and humdity problems. How low do you have to set your thermostat to be comfortable?
If you are comfortable with the electic bill and how it is working then why would be thinking of replacing the system? Even with rule of thumb you are still oversized with a (2) ton system. Did the contractor preform a load cal to determine the size or say you have roughly a 1000 square foot home and it needs a (2) ton.
FYI prices are not allowed here. I can say that the price does not sound unreasonable but still depending on the scope of the work I really can't say one way or another if fair. Comfort does have a value for most people so since the addition is not going to be added make sure this time it is sized properly for the home.
You just have to understand that we see this same thing daily in our line of work. Possiable future additions are just that possiable. When a customer says they are planning on added a addition. I ask how sure? If not 100% sure I suggest to size the home as is and look into other opitons when and if the addition is added. If the customer insists on sizing for the addition I either walk away from the Job or make a note on my estimate that states that going with size that is needed to take are of future addition will be oversized until the addition is added and my company want be at fault if the system does not work properly. The other issue with future addition is this how big will it be, what will the windows, doors and insualtion be etc... To many variables for anyone to size properly without the addition or plans stating all of this!