Well, let's see.

In school we all learned the rule "change the y to i and add es."

Thus, the plural of pony is ponies, of baby is babies.

Why do we do that? Because for some mysterious reason, if we just add an s, we change the long ee to a short i sound. For example, I know a woman named Marlys, which is pronounced MAR-liss.

Now if a word has an e in front of the y, and we followed the rule of "change the y to i and add es," then we'd get:

journey - journeies
valley - valleies

Too confusing! So forget the rule if there's an e in front of the y. The e is a sort of insulation which 'protects' the sound.

PS I don't think monies is the plural of money. Money is something you can hold in your hand. Monies are abstract currencies.