Drop a dime for a nickel

Willing to turn someone in or call the cops even at personal cost.

Derived from 'drop a dime' i.e. 'snitch' combined with 'I would do that for a nickel'

A: "Don't tell him, he'll turn you in"
B: "But wouldn't he get in trouble too?"
A: "He would drop a dime for a nickel"