Kind of hard to say exactly how much it would be worth without more information, but here's a stab:
The base gun itself - 18" barrel, no extension, no sling swivels, no internal mods (I'm assuming) - is probably worth something like $300 - $400, maybe a bit more if it's in really nice shape (brand-new, basically), or a bit less if it's been used hard. There are a lot of police trade-in 870s out there, and
good deals for real basic riot 870Ps usually seem to be in that range. If I remember correctly, a "RS" serial number prefix means it's a fairly new gun, so no real collector value, but it
should have all updated parts, and probably not an egregious amount of "miles".
The sidesaddle looks like a TacStar or some clone. Functional, but not exactly top-end, and prone to literally falling apart if not installed properly (including some steps not given in the instructions). If it was installed by someone who knew the right tricks, and you have the original trigger plate pins and detent springs in case a buyer wants to (or has to) take it off, add maybe $20. If some of the fasteners are loose or missing, or the threads in the baseplate are damaged, call it a wash. Maybe even lower the value by $15 - $20 if you don't have the original trigger plate pins and detent springs (though in that case I'd suggest buying a set to sell with the gun).
I'm not sure how much the fore-end with the integrated light adds to the gun's value. Those fore-ends are $250 - $375 or more brand-new, but for an older one in unknown condition that may not have modern light technology inside - it's tough to say. Definitely a selling point if it's in good working shape, but I'm not sure how much you could realistically ask for it without more information.
