It All Really Adds Up

I don't think its an achievement. Its more of an excuse for blizzard to laugh at players.

Only repairs done with guild bank money will count toward this achievement.

The fastest way to get this is to simply allow every rank in your guild to have a high amount of gold withdrawal per day. Because I can guarantee you if anyone sees they can use guild money to repair their gear, they WILL use it.

However, you must be careful to not set the limits too high. Keep track of how much your bank makes each week with guild challenges and the guildies just looting. Try to ensure the repair amounts are NOT exceeding the amount your guild makes in the run of a week.