Lots of thee type of questions really depend on where you are and the market in that area. In Mass a $500 gun can go for $100 more than it does in NH.
I'd say if it is really clean the P95 itself would be worth, market dependent, around $275. The stainless slide is worth a little more, about $25.00. I can have a new blued P95 sent to my door for about $300.00. That deal isn't hard to find. Gotta add FFL fees.
2 mags (total - 1 extra) are important to me and bump the price up about $10-$20. The holster, that is a throw-in. A holster is such a personal thing that I have a hard time finding them valuable. It is like an aftermarket grip, it doesn't do much to the price unless it is a grip that "everybody" wants. Like any aftermarket item you pigeon-hole yourself with the item. A buyer may not care for it and it may not be of value.