The person you're replying to picked a bad example. Go to Ebay and type "police badge" into the search box. There are a dozen pages of real badges, or passable knockoffs, for sale right now. The cheapest real badge I saw on there was only $15.
You may not believe the tin star, but what if someone flashed this one at you on a darkened street? It's yours for $6
http://cgi.ebay.com/Vintage-Style-SPECIAL-POLICE-Badge-Nickel-NIB_W0QQitemZ200049100992QQihZ010QQcategoryZ929QQrdZ1QQcmdZViewItem?hash=item200049100992
This fine example is even nicer, and can be yours for only $20. Sure, you'll see that it's fake when you read the words, but how many people are going to read the text on a cops badge?
http://cgi.ebay.com/Obsolete-BILLY-REEVES-RUSHVILLE-POLICE-IND-Police-Badge_W0QQitemZ160054507437QQihZ006QQcategoryZ929QQrdZ1QQcmdZViewItem?hash=item160054507437
"You may not believe the tin star, but what if someone flashed this one at you on a darkened street?"
Those examples that you provided are more realistic, but still would be obvious fakes at close range in the daylight. I concede, however, that in the dark at a distance of more than a few feet, any fake could look realistic enough to fool someone.
I just couldn't resist ribbing the previous poster because his example was so obvious. But, in the dark at distance, who knows?