A pair of Zotac AMP! 980Tis, about a year ago, at $650/piece, if you consider them as a unit.
If not, my previous monitor, a Planar PX2611w 26" 16:10 1920x1200 IPS display, was $800. My current display, a Dell U3415W 34" 21:9 3440x1440 IPS, was around $700.