204.1. Base Costs: These costs are the most fundamental costs that must be paid to finalize a card or ability.
204.1.a. On cards, the Base Cost is found in the upper left corner of the card.
204.1.b. On activated abilities, the Base Cost is the resource or instruction written before the ":". If there are multiple choices for costs to pay for an activated ability before the ":", the Base Cost is whatever cost the player chooses from among those options. Example: Legion Marauder is a unit that reads in part "[Empower] -- [1] or [C]." This is functionally short for "[1] or [C]: Empower me. Play only if not empowered." When finalizing the ability to the chain, the controller of the chain item will choose one of the base costs to pay. That is the base cost of the ability.
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.