It isn't the union that makes the cost of police high, its that they're a select group: physically fit(when they're hired!) yet no criminal or antisocial record(surprisingly rare), and fairly intelligent. Add the expensive training that goes into policing and you get what you pay for.

None of what you said is factually accurate. We don't pay enlistees in the armed services high salaries, and they're usually physically fit. No criminal record? Depends on the department since each sets its own standards. Expensive training? Many departments have academy training that's 8 weeks. If a department uses a longer 6 month training cycle, the average tuition cost is around $7K.