Once again, missions are not about making profit, they are about increasing the faction (political control) of the faction you do the mission for, and your reputation with that faction to advance your rank with one of the big 3. If anything, the effect on faction influence should be increased so that after doing 20 missions in a row (Like I have done in a system near liaedin...volungu i think its called) for Patrons Principles, you can actually see a change in faction influence.
There is already way too much credit reward in the game as everyone wants their <insert ship here> yesterday. This game is NOT COD. Elite has always been about working hard long term to achieve great things, but this outcry for more and more credits is in danger of making this an arcade game.
So to summarise, leave the cash rewards alone, but if you must increase the effects players have on the universe directly through these missions so you can see change.
InMemoriamSirocco also has a good point, create some missions that benefit players that have done the work and have bigger or more powerful vessels with similar increase in risk and also non monetary reward(bigger faction effect)