Underground shelters would be quite a bit cheaper than a standing army of rockets capable of shipping all lunar colonists home. They work for Mars as well.

Of course, given your penchant for spending way more money than necessary to solve technical problems, you might want to consider whether your plan would even work.

a) Cosmic rays rain down all the time. If you need the safety of Earth's magnetosphere to protect you from them you have to stay there.

b) Solar flares take a matter of minutes to reach the Moon. Exactly how quickly do you expect a fleet of spaceships capable of transporting a colony to Earth to prep for launch let alone make the trip?

better luck next time,
Mark