Multiple instances of bloodthirst are cumulative. For example, if you control Twins of Discord and a colorless creature with bloodthirst 1 enters the battlefield under your control, that creature will enter the battlefield with three +1/+1 counters on it if an opponent has been dealt damage that turn.

The mana value of a face-down creature is 0.

The mana value of a token that isn't a copy of another object is 0.

If a creature on the battlefield has {X} in its mana cost, X is 0 when determining its mana value.