Are you just using it for on-set monitoring? Does it need to be color accurate?
If you're just using it for monitoring focus/composition or for rough preview then any HDMI compatible screen will work. Once you run your cable from the camera to the monitor though it'll tie you down to that spot (fine for some type of productions....not viable for run 'n gun shooters).
If you need a color accurate monitor for on-set then you're looking at spending quite a bit more, but you can have a look at FSI monitors, TV Logic, or Sony Professional monitors. With color accurate monitors you're looking at a starting price of around $3K+.
Check out the Swit s-1221 series (F, H) of 21.5" monitors. I'm actually a reseller in Los Angeles. But I started as an end user with their 7" for shooting. I liked them so much I asked if I could be a vendor.