Obviously, it depends on size of the aircraft, how far you want to go, does the airline have a station at either end (or will they be paying for ground handling), and when the charter is.
A general rule of thumb is if you are willing to fly in the middle of the night when the airline is not offering flights, the price will be less.
to GRK (1161 miles) I would often see prices in the $27,000 range for a 137 seat WN
737-700. That works out to about $200 a seat, or a little less than 18 cents per seat mile. I would see similar pricing (on a per seat basis) from Miami Air and North American for 727-200s and 757s.