The Sox, like a lot of teams - probably did not expect the market for guys like Moose to run dry as much as it did. It also hurt Moustakas that he frankly has not actually been that good - last year is what the bet is, and defensively he is more "okay" than Machado-like at 3B.
Given that he was there effectively as insurance, $6.5M for Moreland is fine. The team should keep him away from LHPs, but it is a good problem to have.