Strange, I just stopped by a local dealer today that was asking $529.00 for that exact model - the only reason I noted price was because of the 590A1 I'd recently bought. So unused, LNIB for $399 should probably be a good deal about anywhere.
But since I'd paid about that same price ($529) for the 590A1 Blackwater Edition, which comes with the X-S Systems top rail, ghost-ring rear sight, white ramp front sight, tri-rail fore-end and speed-feed stock ... hmm, tough call.
I surely favor the Mossbergs over the Rem 870 Police due to the far more logical control locations (action release at rear of trigger guard, tang safety vice action release horribly located in front of trigger guard on left side and stupid cross-bolt safety on rear of trigger guard) ... so if that seems a decent price locally for a virgin 590A1, I'd surely snap it up and pay the fifty bucks or so to get rear ghost-ring and front ramp sights put on ...
Of course, since my employer has long issued the 870 Police Magnum, I'm considered a heretic in local circles (I'll grant the 870 is built like a tank, totally indestructible and far easier to administratively unload), but if I have to use a shotgun in dire circumstances, give me a Mossy ...