There's a saying in Equestria that "Keeping the harmony doesn't come cheap." Many owe the saying to the countless ponies—and the Elements of Harmony themselves, no less—who have sacrificed so much to keep a lasting peace in Equestria against all threats.

Twilight just didn't anticipate a more literal component of this... or rather, a monetary one. On the bright side, she now knows to never trust her friends with money again.