With triggered abilities that allow you to pay a cost to get an effect, you are only allowed to pay that specified cost once unless stated otherwise. An example of this would be with the triggered ability of Mirrorworks. When a nontoken artifact enters the battlefield under your control, Mirrorwork’s ability will trigger. When this triggered ability resoles you are given the option of paying two mana just once. If you do, a token copy of the artifact triggered Mirrorworks ability will enter the battlefield under your control.