Do I have to pay Power for Weaponmaster?

Yes, you pay Power when using Weaponmaster — just 1 less than the equipment's Equip cost.

Weaponmaster works by taking the equipment's Equip cost and reducing it by [A] (1 universal Power). So if the gear's Equip cost is 2 Power, you pay 1. If it's 1 Power, you pay 0. If it has no Power cost component... you can't attach it via Weaponmaster at all (Rule 821.1.c.4).

Also, any non-Power costs on the Equip (like "kill a friendly unit" on Blade of the Ruined King) are not reduced — you must still pay those in full (FAQ 5147).