I've been lucky with artists so far, however I've never dealt with Ale Garza. In most cases I've either not paid anything upfront or paid a small non-refundable deposit, 10-15%.
A small non-refundable deposit shows the artist you are serious, but also lets the artist realize they have to finish the work if they want the rest. In the event of paying nothing upfront the artist knows it's entirely up to them to deliver, and if the client gets tired of waiting, they can sell the completed piece with no guilt.
For artists I dealt with successfully multiple times in the past, I don't mind paying upfront.