I have an 870 and a Berratta Silver Pigeon o/u. Personally, i dont like side by sides, i find them a little unweildy, sadly, the closest you might get to an 870 on cost is with something like a mossberg semi auto, although the earlier suggestion for an old weatherby is a great idea. The Citori is a good solid tool, and can be bought used from just about anywhere for a decent price. But a used berretta is still going to cost a few BTU’s (Basic Toy Unit = $1,000). It seem silly that a high tech handgun will cost $600 but a halfway decent skeet shotgun starts at $3k, and QUICKLY goes up from there.
My suggestion, check for estate sales and see if you can find that mythical deal with the little old lady.
Or, to make yourself a bit sad, but better educated, check out these two websites:
www.joeletchenguns.com (used)
www.pacificsportingarms.com (pre-owned)
One last thing, trap guns and skeet guns are radically different. Skeet guns are more versatile and are what i would recommend. You can distinguish between them because the rib along the top for sighting is raised up on a trap gun, sometimes raised by a LOT.