Well if you are able to find a second or third person to break down the trip cost, there are air taxi operators already doing business who will match airline prices. I know, I just ran several trips through the pricing function on one air taxi operator's website I am familiar with in Lawrenceville, Georgia. Theirs is for an SR22 with 3 passengers. I got $250 a seat for a 250 mile trip. On a Mustang you could break down the cost by adding a fourth or fifth passenger. That Mustang quote you have there should break down into 5 equal parts. I will venture a guess that the airports it serves are not served by a regional airline anyway.