Estimated I remember reading was $1500 - $2000 per ton (?) of heating / cooling capacity (I think 80K btu is going to be alot more than $2000 - $3000).
Trane is supposed to be very good, as is Carrier. Lennox replacement parts are supposed to be really expensive.
And I've seen it recommended that you get you should buy from and competent and qualified installer, who will do required preventive maintainence too.
Personally, I wouldn't use Sears, Home Depot, Lowes, etc. because I believe they just subcontract out to whoever is willing to work for them for least money. Why not go to independent authorized shop?
Plus, if you want to get optimal operating efficiency, doesn't dealer have to carefully look at your house, rooms, windows, etc. to get a very accurate estimate of how much heating cooling you really need?