There is a pool that's shared. until your at the right levels your % drops for Xcoin champions can be awfull. 1 coin start out as most common and drop down to become most rare at level 9.
If your rolling for 2coin units. levels 5 and 6 have the best drop rate. 35% for 2 coin . Level 3 and 7 are 30%. Then 25 then 15.
If your looking 3 coin units are 0-5-15-25-30-33-35-35.
Remember that this is just drop rate, the individual champions are out of a pool so odds change depending on enemy picks. They eat up options that your not building? more likely to see. They eat the ones your building? Less likely drops.
I will try to screen cap some footage to show ya.
Edit. Here. https://www.youtube.com/watch?v=NuSxAkFCfYU&feature=youtu.be