Historical Rules · 30 September 2011
Rule 602.2b
Official source text from the 30 September 2011 Rules release
The remainder of the process for activating an ability is identical to the process for casting a spell listed in rules 601.2b-h. Those rules apply to activating an ability just as they apply to casting a spell. An activated ability's analog to a spell's mana cost (as referenced in rule 601.2e) is its activation cost.