I'll try that the next time I fill up my car. "Please, sir, can I have it for a bit cheaper?"
No, you can't make fuel prices any cheaper. You can
choose to fly more economical planes though.
I don't know what you've been flying that is causing you to go bankrupt though. The fuel price really isn't that bad. There are times in some games where it's more than double what it is in the EC game right now.