See this recent thread: https://forums.adobe.com/thread/2420222
In particular, see post #6 for things to look out for. This is what it's worth paying for - and where manufacturers often cut corners to cut costs.
In general, 24" at 1920 x 1200 is the best price:quality ratio. Larger panels and/or higher density drives the price up fast with no real quality or usability benefit.