What determines the cost to respec?

Solution 1:

Respec costs are tied into your character level and the number of previous respecs you've had. The difficulty level has no effect on the cost.

Respec cost = 256 * (Character Level) * (# of previous respecs)

In the exceptional case where the number of previous respecs is 0 because you haven't ever respecced before, plug 1 into the formula. Thus, the very first two respecs you do, if you're the same level for both of them, will cost the exact same amount.

I've tried at two different fate weavers, and it didn't affect the formula. The Mercantile skill also had no effect.