The description of the Usurp node of the Soul Feast tree states that “Modifiers to the amount of ward granted by Soul Fragments also apply to the amount of mana gained from this node.” This does not appear to be true.
The base mana gained from the node is 6, and best I can tell, that is all that is gained. The prerequisite Wrest node with 4 points gives +340% ward, so with that mana gained should be 26. The Soul Seeker node gives +3 ward and +3 ward per 25 Int, so that would be 15 ward with 100 Int. Then the Bones of the Accursed node give +25% more ward per curse, and you have to have at least one curse applied for the skill to work. So it should give over 50 mana, and it appears to give 1/10th of that.