Is the Cortex M0 really low-power?
You aren't entering the lowest power mode.  Take a look at AN11027.

Regular sleep mode yields single digit milliamp consumption, as you have seen.

Deep sleep mode is in the single digit micro amp range.

Deep power down is a few hundred nanoamps.

Also be careful of things in the surrounding circuit which can steal power (or even supply it, invalidating your measurements).  These could include connected communication or programming interfaces and pullup/pulldown resistors working against an asserted signal.