If I've got up to $400 for a monitor, which one should I buy? I'd really appreciate guidance on this.
What kind of experience are you looking for? What's the rest of your setup?
Do you want the best 1080p experience? Or the best 1440p experience? or even 1600p?
There are a lot of options at or around that price point.
There is a perfect pixel Yamasaki 30" 2560x1600 display on ebay for $399 regularly, free shipping.