*The prices that are listed for paint and sundries will vary and also are purchased at a discount by the contractor The total cost for painting an interior will run about $1.02 per square foot (about $390 for the for a 12x12 room) of wall space being painted.A professional painting contractor will be able paint an average bedroom like we listed above in only half a day.84' x $.70 = $270 (268.80) Paint for this room will run about $.20 per square foot of wall space. This will add about $70 to the price above if using a Benjamin Moore high quality paint. Then, sundries, or supplies will be added. They typical cost for this will be about $50 ($.13 per square foot) for a typical room.