The federal monitor appointed to oversee the NYPD’s stop-and-frisk tactics may cost millions of dollars, experts say.

Smaller cities like New Orleans and Seattle have spent between $1 million and $2 million annually for similar oversight — for departments 20 times smaller.

“I think the bottom line is it’s not going to be cheap,” said Samuel Walker, a retired professor of criminal justice.A federal monitor charged the city as much as $150,000 per month between October 2011 and February 2013 to keep an eye on hiring at the FDNY, which is about one-third the size of the NYPD.