I'm not sure his defense is that good. If it was, I doubt he'd be in this situation, but let's assume it is very good. By adding him to the 40 man roster, his actual added cost to Henry would only be the tax. 20% on $11M is a pretty good deal for a good 4th OF'er. If we go over the second level, it's 32%. That's not bad either.
The issue arises, if we go over the luxury tax 2 or 3 years in a row. The tax goes up and up, and if Castillo struggles, the cost becomes steep.