When is damaged calculated when using a skill?

I'm considering making a build using Wild Strike of Extremes, which has a life, mana, and ES cost. Was considering using Trickster for Soul Taker which gives energy shield overleech. Assuming I'm using Energy leech support and I have enough leech to basically instantly be full again before the next cast would I ever benefit from being full on energy shield?

Is the cost of the ability pulled before damage is calculated, or is the damage calculated before the cost to cast has been done? I'm not sure how to test this or I would do so myself, but it makes a fairly large dps difference in the build. Anyone have an idea of how to test this? Or maybe a developer that knows how the calculations would work?