The price is really based on the quality of workmanship and professionalism one is looking for.
The price usually varies from $1.25 - $4.5+ sqft for a complete paint job including materials. Complete meaning painting of every paintable area. The average cost per square foot for painting the interior being about $2.5 - $3 sqft.
The low price of $1.25 sqft will usually include the most inexpensive paint materials, none to very little prep work, and mostly a sloppy job (visible miss spots or holidays, color bleed-through, paint drops on the floor, very little cleanup).
A bad practice I've seen over the years is someone coming in at this price, promising good materials, and "watering down" the paint. I've heard of people putting as much as 1 gallon of water for every 2 gallons of paint. No matter how good the paint was to start with, that water just destroyed the formula and probably made it worse than the inexpensive paint would have been.
Looking at the higher end of the spectrum, at $4.5+ sqft, you're looking at very high quality materials being used, very detailed prep work, and a professional, licensed, insured, and trained crew.
Condo / apartment painting is usually a little less because there aren't as many separations and hallways / stairways, etc. They usually start at $0.80 - $3.5+ sqft, averaging at $1.25 - $1.75 sqft.