I've had pretty good luck over the years finding good condition trade-in or second hand P series pistols. I lurk over on Sig forum some and they often link vendors with caches of trade-ins and the like. I've purchased 4 or 5 of these in recent years. I think the most I paid was $465.
A couple of tips:
-Look for DAK models as they're often quite a bit cheaper. The parts to convert them to DA/SA, if you so chose, are readily available and run about $150 or so. There is a contingent of folks around who prefer the Kellerman (DAK) design. I've sold a bunch of "DAK parts kits" and usually at least break even.
-Buy less sought after calibers (.40 or .357 Sig), buy a 9mm conversion barrel then sell the original. Or don't and keep it. But you can generally get them for less than the euro-pellet chambered ones.
Keep an eye on these sites. They stock pre-owned and trade-in Sigs. But they tend to go pretty fast when they become available. A few of them have email notifications for particular listed used models. Take a gander around. There are some in stock at a couple of them now...
Bud’s gun shop
Aim Surplus
Osage County guns
Impact guns
J&G sales
There are more too. Google is your friend.