Farming herbs is slow, but definitely cost efficient. I bought 100 irit seeds for 5k. Just be patient, it'll save you lots of money. Another method for saving money is making Serum potions, Tarromin + ashes, both are very cheap and will get you 50 herblore xp. Only problem is the serum is pretty useless after you complete the quest, and you have to empty the vial.