Do willow trees, and if you use apple trees along with it, you save money on the willow payment. It's my method, and doesn't exactly break the bank either. It's also really fast, I've gotten about 60 farming levels in less than a month by doing trees. Of course, also do herbs but you have to be a lot more diligent in looking after them. It's just easier to get the stuff for trees, to be honest. Make sure you keep in mind the timers and how often things grow, and you can just come back once in a while to reap the rewards and replant whatever it is you're doing.
If you combine farming with slayer or other money-making methods, you'll at least break even without sacrificing speed. You should also do quests which give farming experience as a reward, the early fairy ones are especially good for this.
Good luck in training! It's not a difficult skill once you get into a rhythm, it just takes perseverance and a bit of time management.
