Yeah, I forgot to mention that I'm flying as a merc. I like this mode; it adds an interesting demension to the campaign.
Anyway, it seems you're right about aircraft range being an issue. I did some testing with the F-5A's NormalMissionRadius and MaxMissionRadius values, and found this--
Radius 798/898 : $65000 (original)
Radius 598/498 : $60000
Radius 398/298 : $57000
Radius 198/098 : $54000
That's as low as it will go from MissionRadius changes. Price seems to be unaffected by Celing, CruiseSpeed, ClimbSpeed, and most other FlightControl values. However, MachLimit has a significant effect. With MissionRadius still at 198/98, I changed the MachLimit value from 1.40 to 2.40, and the F-5 jumped to $71000.
I don't know if any other modders will find this useful for anything, but I think it's interesting to figure out how game mechanics really work.