I just traded an almost new 11-87 sportsman for a new 870 Police. It is the model with the 18.5" barrel, +2 extension and raised bead sight. I'm very excited to get it, as far as cost goes the police models seem to cost a fair bit more than an express with the same specs, what makes the police more money or a better gun? I know I like the fact it comes with a fixed improved cylinder rather than a cylinder bore.

The 870 express versions are only finished to a certain price point so that folks can buy a new one at the big box store for $300.00 out the door.
The 870 Wingmaster along with the Police model is given more attention to quality and finish.

I have read that some internals are different, possibly some springs. I did see somewhere on the Remington web site that the receiver is media tumbled to remove burrs and the guns are test fired and function tested. I was just curious as the police models are more expensive and I find it hard to believe it costs over $100 to do some inspection and deburr a few parts.

The finish is about the biggest difference. Police models usually get a high-quality Parkerized or blued finish, while most Express models have a less-expensive, lighter-duty oxide finish, and are rougher out of the box (they smooth out just like any other 870 as you use them, it just takes a little longer).

Aside from that, Police 870s have heavier-duty stocks (at least for models with polymer furniture), and short fore-ends that are compatible with cruiser racks. Some tactical/HD Express models have the same fore-ends, but the stocks are usually cheaper (though still perfectly functional). Police guns also have a couple heavier springs inside to give them firm trigger pulls and such, but those springs aren't necessarily better; they're just intended for combat instead of hunting or sporting. In any case, they're just a few bucks each, and will drop into any 870 if desired. There are a couple other minor cosmetic and materials differences, but those don't really change anything on a functional level. The finish, furniture, and heavy springs are what really make an 870P what it is.

If you take an Express, strip it and Parkerize it, and swap out a couple internals - maybe the stock, you'll essentially have a Police. A lot of the high-dollar custom combat 870s out there started out as Expresses because the folks who build and sell them usually want to add their own special finish anyway, and they know that an Express is the exact same gun and action under its lower-end finish.