DML makes a good point about cost, but you can take a look here and judge the better deal.
Here's a run-down of approximate costs for my 870 Express project:
$210 870 Express with 28" barrel
$115 20" IC barrel with rifle sights (Cabela's)
$100 Insight Technologies M3 tactical light (Botach Tactical)
$15 Insight Technologies M3 tactical light bracket (Botach Tactical)
$45 Choate mag extension kit (extension, bracket, follower, spring) (Choate)
$30 Remington factory synthetic stock set (eBay)
$25 Tac-Star 4-shot side saddle (Wholesale Hunter)
$4 Remington old-style safety (Wilson Combat)
$10 Pachmayr slip-on recoil pad (Wal-Mart)
$1 1" nylon sling (eBay)
-----------------------------------------------
$555 = Subtotal
- $100 Sold 28" barrel (TFL)
- $40 Sold hardwood stock and fore end (eBay)
---------------------------------------------------------
$415 = GRAND TOTAL
Of course, I had fun tinkering with this gun and changing it from its original configuration. Part of the fun was finding add-ons at good prices and installing them myself.
Also, a number of the items listed above would not come on the Police model (tac light & bracket, heavy duty follower, recoil pad, sling, side saddle), so you'd have to add those costs onto a Police model as well. For just the stock gun with mag extension and old-style safety, I was looking at a lot less than the Police model runs normally.