The average cost to paint an interior is about $0.45 per square foot per coat of paint. Factors such as high ceilings can increase the cost slightly. Another factor that can increase the cost is the condition of the substrate being painted.n the Chicago area prices come out between $1 - $4 per foot to paint an interior room. Many variables come into play for this. If someone is looking to just paint the walls, … no ceiling or trim, they're looking at the $1 sqft.