I know it's cheap to buy but I was wondering...
Why not make your own yeast?
As an amateur cook, I've gone so far as to make a batch for good sour dough bread - warm water, flour and a touch of sugar and let it sit in a window sill until airborne fungus likes the mixture and a colony starts. In a few hours (according to some) or days (in my case), homemake yeast with added mixture of miscellaneous fungi that give a sourdough it's rich taste.
Saying that, it might be an intresting CNR recipe for the press (because the ingame item is a pressed tablet, not a cup):
1 Wheat flour
1 Sugar
1 Bucket of water
Yields 5 baker's yeast. (Just a guess on the number but it has to make the risk worth it)
The odds would stay, if it's possible, at 50% for all skill levels - it depends on mother nature, not on your ability to mix flour, sugar and water. There wouldn't be any crafting XP for it for the same reason but maybe a small XP bonus for getting lucky.
I've never used nor made real brewer's yeast but, just to be fair with the brewers:
1 Rye flour (or other brewer's grain, maybe barley)
1 Sugar
1 Bucket of water
Same baker's yeast rules would apply.
The rewards would have to match or defeat the inherent risk of messing up - 50 gold for 5 tablets of baker's yeast or nothing for a 50% of 5 tablets?
I don't know, just a suggestion.