In my opinion, taking a cheap way to train almost any skill is a waste of time. With my logic, you can spend the time you save with a faster, more expensive money to recoup your extra losses (and then some, usually).
If that makes sense to you, stun-alching, bursting -> barraging, and alching at the mage training arena are your best bets. Just look around at prices/alch prices to figure out what to alch if you're stun-alching.