March of Burgeoning Life's target must have a mana value less than X. A creature with a mana value equal to X is not a legal target.

The mana value of March of Burgeoning Life while it's on the stack is the value chosen for X plus 1, no matter how much mana you actually paid to cast it.

For example, say you wanted to choose a target with mana value 3. You need X to be at least 4. You could choose to cast March of Burgeoning Life by exiling two green cards from your hand and paying , by exiling one green card and paying , or just by paying . If you had a reason to, you could even choose a greater value for X.

You may exile more cards than necessary for March of Burgeoning Life's first ability, but you can't reduce the mana it costs to less than this way.