Second, why aren't you already at a different job that pays you more instead of hanging that over your manager's head?
Third, don't underestimate how many other good developers that are out there, looking for the next job. It might not require 6 months of training for the next person, who could be as competent, or even more than the person that's being replaced.
I'm not sure what kind of company you work for, but 6 months of training is ridiculously long, unless there is a lot of institutional knowledge that's siloed amongst a small subset of employees - which should never happen in the first place, if your manager was any good.