I'm just curious about the decision making process for this.
Like why Udyr's stances reduce costs as they increase in rank, while most other abilities increase costs.
Or why something like Braum Q's cost is 55 / 60 / 65 / 70 / 75, but tahm's is a flat 50 mana.
Is it totally dependent on how the CD scales with rank or are there other factors?
its dependent on the ability. It's really simple, "should mana (or energy) gate the hero more or less or the same as as the game goes on" there are very few cases of mana being a larger gate as the game goes on, due to mana pools increasing with level. Usually riot simply tries to make them stay the same relative to total mana. This means that a hero can cast some abilities more as the game goes on, but they will always be stuck with "can only cast X ability Y times".
I was lucky enough to see the Design and balance team in action viewing these very issues. This is how it is decided:
http://moneystepper.com/wp-content/uploads/2014/09/Monkey-darts.jpg
