If really want player do more different mission, maybe make something like a chain bonus.
Like you help Martin finish a chain of mission in a row, you got a bonus payout, or you finish a chain of mission, you can do a secret mission which is high payout and high difficulty.
At least you got some reason to do low payout mission in that case.
That's actually a pretty good idea... I'd suggest it be sort of like how the "Clear Out" award is achieved:
When joining online, you get an email sent to your phone (using the recently added email icon). Once accepting this, you trigger the start of a "playlist" of random missions, one of each from every contact:
So, 6 missions and if you complete them in succession, you get a bonus.
All of the missions in the "playlist" from one contact would work, too... Just figured that a mix of contacts would be better, so chances are less that you get 6 low paying missions.
Should be pretty easy to implement, from a programming standpoint, I would think.