honestly herblore should be a hard skill to get to 99 including expensive, make it cost 300m to get to 99 or so. secondary's are always more an herbs are less
Make it so Herblore is profitable
Prayer Potions,Restores,Brews ect
and Have monsters drop low amounts of secondaries ect.
Herbore can be another way of making GP but don't make the exp toooooo fast, Have it at a decent rate to where it's worth it in the long run