I assume we're talking about the several varieties of police trade-in M&P that Bud's has currently.
If you're looking at an unissued model (which Bud's seems to always sell for $399), I agree that you may as well buy new. You'll need to replace the sights and pay for a smith to install them, and there's your hundred bucks for a brand new gun anyway.
If you're going with an issued gun, you can sometimes get those as cheap as $330 or so. Yeah it's a used gun, but with that route, you're actually saving money, even after replacing sights. I guess it all comes down to, how much do you wanna save 50-75 bucks?