I have the Moss500 with factory Ghost Ring sights and they are nice (and installed) for $50. I purchased the 6-shot variant with blued finish, hi-visibility front sight and it works very well for patterning buckshot or slugs at longer ranges.
If you're definitely going to be using the shotgun for perimeter defense, the factory Mossberg ghost rings are very effective and add considerable value and flexibility to the gun, IMO. I shopped around for a shotgun with factory installed sights and this is what I ended up buying.
On top of that, the factory Mossberg ghost rings are easily adjustable at the rear for windage and elevation for a simple adjustment in the field. Only slightly off at the factory, and a quick adjustment had them shooting dead on. I love mine.