I *think* that the rewards for some of the missions are credits & when they are all completed they add up to 40 credits so you don't actually 'waste' credits on solo missions.
That's incorrect Coldberg
total sum of prizes for the unlockable solo:
1600ctz, 1500bp, 80 tickets for each set
at least according to mission wiki
I've completed 60/60 solo missions & my rewards in total were: