What was the salary of a police officer in the 1920's?

I'm looking for the same thing. Depending on the department, it looks like the rate of pay for police officers varies, and there were also differences between the officers themselves.

In one document I've found from 1920, the minimum salary for a police officer is $125.00 per month. I'd go from here; most salaries should fall somewhere around or above this rate, probably more for larger cities.

So: ~$125.00+ per month.