When you play a card, its numeric cost starts at its Cost Number. As you pay the card’s additional costs, you may encounter instructions that say “Pay [number].” Add [number] to the total numeric cost of the card. Once you’ve fulfilled all of the card’s additional costs, you apply cost increases. Then, apply cost reductions. The final number is the card’s numeric cost.

Abilities are handled in the same way. But since abilities do not have an inherent Cost Number, any ability without a “Pay [number]” instruction in its cost is considered to not have a numeric cost at all.