I might be out of line with current market prices, but I think that's easily $100 too high. If it was $225 it'd be a good deal. If it was under $200, well, you'd have to buy it.
I mean, you can pick up a brand new SP-101 for about that price, can't you? And a GP-100 for about the same. I haven't lately seen the huge numbers of police trade-in Ruger Security Sixes that used to be around, but I'd bet a perusal of the Gun List or even Shotgun News would show similar revos for about $100 less.
Good gun, though. Good luck with it!