Another good idea.
How about having missions for example you have a Gigantic Forest Fire for which you could get the maximum Credits off 100,000 if you complete it in time.
However if you got to complete the mission say by 20 minutes and you get the full amount but the next time you don't do it in 20 minutes and it takes you 30 minutes you should on get a percentage of it.
20 minutes you get 10K credits
30 Minutes you get 7,5K credits
40 Minutes you get 5K Credits
Does that make sense?.
It just makes the game more interesting and a challenge and hopefully might attract new players.