I'm not sure how much playing with fares does (unless you have competition). Raise the price and you get less people, lower them and each person gives you less money. If your planes are actually full, then you definitely need to raise them, but thats not a problem you have right now. Your fleet of 1 Comet is actually probably doing much more damage to you then the one Ambassador. Have you checked your commonality costs? How do they compare to your aircraft's profits?