Even the prospect that they might make money someday scares the front office, apparently.
They signed him to an extension earlier this year- $15 mil for 3 years, I think- so they already knew what he was gonna make. This looks like a pure baseball trade- McLouth, while a good player, is not extremely difficult to replace (especially if his defense is as bad as some say). I don't know much about the prospects but sounds like all 3 have a chance to become good - probably not great - players. And sure, they save a little money.
By the way, I thought the name was "Galvine"?