Rule 204.1.c
On Triggered Abilities, the Base Cost if it is present is the Costs within Instructions that appears as the first part of the effect. It must be paid to finalize the triggered ability. See rule 204.3. Costs within Instructions for more information. Example: Overzealous Fan is a unit that reads “When I defend, you may kill me to move an attacking unit to its base.” In order to finalize the ability to the chain, its controller must kill Overzealous Fan.