Actually, Senshi powers HAVE to be bought in smaller pieces because all of their powers at once would likely be far more than just 90 MP. Each power regain represents one solitary ability (or abilities that are small variations on the same central effect). Psychic powers would be one ability. Super strength would be one ability. Flight would be one ability. The costs for each of these would depend on their potency.

If your character had just one power, though, that was all the same theme (lets say, a super powerful pyrokinetic) it could be bought in steps, such as buying very weak control over fire, up to medium, up to max. But when her powers include multiple effects, then each of these have to be bought separately.