K.18.1- The "mana cost" of a card is the mana specified in the upper right hand corner of the card. [CompRules 2001/07/23 - 203.1]
K.18.2- If a spell or ability specifies that the mana cost be paid, it means the actual colors and amounts specified in the mana cost. [CompRules 2001/07/23 - 203.1]
K.18.Ruling.1- The "mana cost" does not include any additional costs in the card text, or any additional costs imposed by other cards. Those apply to the "play cost" (see Rule K.20). [CompRules 2001/07/23 - 203.4]
K.18.Ruling.2- When a spell or ability allows you to cast a spell without paying the mana cost (or by decreasing what you have to pay), the mana cost is still considered to be the full amount even though you did not pay it. [D'Angelo 1998/02/03] See Rule K.20.
K.18.Ruling.3- When a spell or ability allows you to cast a spell without paying the mana cost (or by decreasing what you have to pay), this will not affect costs which are not part of the mana cost. [WotC Rules Team 1998/02/01] For example, Aluren will not pay for the extra cost imposed by Gloom.
K.18.Ruling.4- A permanent in play by an effect such as Animate Dead still has a mana cost as printed on the card, even though that cost was never paid. [D'Angelo 1998/02/03]
Note- Lands have a mana cost of zero. See Rule K.15.7.
Note- Token creatures have a mana cost of zero. See Rule K.25.5. See Rule K.25.Ruling.6 for an exception.
Note- See "Converted Mana Cost" (Rule K.8) for more rulings on costs.
Note- See "Play Cost" (Rule K.20) for more rulings on costs paid when casting a spell.
Note- See Rule K.28 for information on 'X' in the mana cost.