The only time I use a sq/ft price is when bidding new construction, homes and commercial. The price ranges from $.50 on up, this is on the wall sq/ft.
A good price for your home depends on many factors.
1) Your Location – Where you live has a big impact on the cost. It all comes down to "cost of living". The more it costs to live in a area the more goods and services costs.
2) The Amount of Furniture – Moving and covering things takes time and time is money. The more stuff you have that a painter has to deal with the more your paint job will cost. You can save some money by moving some items yourself.
3) Repairs – You mentioned your walls are in good shape. That is good. This will save you some money.
4) Trim and Doors – If these items need repairs or some kind of refinishing this will add to the cost.
5) Colors – The amount of different paint colors and the intensity of the colors does affect the price. Generally, the more colors the higher the cost. Also some colors require multiple coats for perfect coverage.
I can't say for certain how much you should pay. There are many variables. Try to get bids (painting proposals) from three different painters. Have each add to their printed proposals the materials they will use and how the work will be performed. Often going with the cheapest bid isn't the best thing to do.