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Mystic Forge combining can make absurd amounts of money. Yesterday I ran out of skill points converting crafting materials from one tier to the next one up, at a rate of literally 3 gold per minute. That's how long it took to buy materials, combine in the forge, and sell for a huge profit.
The key to playing the mystic forge is to quickly look up the validity of combining materials. So you need to memorize the following formulas:
Tier A -> Tier B
5x Dust + Philosopher Stone + 50x Tier A + 1 Tier B < 16x Tier B
I use 16 because that's the approximate amount of items you will get back per attempt. If you want to tip the scales further in your favor, then you will want it to be closer to 12 or even lower of Tier B is greater than the other mats I listed on the left side of the equation.
That's one method for combining, but you can also forge together things like ore, wood, and cloth. Those have a similar but slightly different equation.
5x Dust + Philosopher Stone + 250x Tier A + 1 Tier B < 25x Tier B
Know the formulas and you will be able to pounce on these kinds of opportunities as they occur naturally in the economy or after unforeseen changes to the game take effect.