Can you provide a more precise price ceiling? Also, do you only want to buy new, or are you open to looking at used guns as well?
If you're going to buy new, then I recommend the Stoeger Cougar 8000F as the best value in a full-size 9mm pistol today. Stoeger is a subsidiary of Beretta and currently produces the Cougar (a terrific $650-700 gun when it had "Beretta" stamped on the slide) on Beretta tooling. I've seen them for $365 new in Austin and (currently) as low as $320 new online. It's a very refined, accurate, reliable, durable gun at a great price. And the metal frame is a plus to me personally.
In the past I'd have given a more enthusiastic endorsement to the polymer Sig SP2022, but prices have gone up a fair amount, and it's much tougher to find one under $400 than it was a few years ago. Additionally, what made earlier prices a real steal was that the SP2022 was exclusively made in Eckernförde, Germany and was thus immune from the corner-cutting and QC issues that Ron Cohen's vision has produced at Sig in Exeter, NH. That's no longer the case, and that's more important to me than the rise in price.
You could once have a quality DA/SA 9mm with decocker for under $300 in the polymer-framed Ruger P95, but it's out of production, and I doubt any residual unfired specimens are selling for that price, though it might be worth your taking a look. It's not a particularly refined gun, but it's perfectly reliable, and it's built like a tank.