If you add your own credits to the purse when setting up a CC tournament, those credits will be paid out 100%.
If your tournament has an entry fee, 80% of those entry fees will be paid out.
So, for example, if you have a tournament where you put in 1000 credits and the entry fee is 10 credits and 5 people enter (50 credits in entry fees), a total of 1040 credits will be paid out.