2022-02-18 : 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 {G}, by exiling one green card and paying {2}{G}, or just by paying {4}{G}. If you had a reason to, you could even choose a greater value for X.

2022-02-18 : 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.

2022-02-18 : 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.

2022-02-18 : 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 {G} this way.