The glassy screen of your MBP isn't going to be color accurate. Any monitor under $200 is not going to be color accurate. If you're looking at under $500, just get the largest monitor you can afford... I hear Dell's are good.
Dell 2209WA. Best you can get at around $200 but is currently out of stock. Not going to find another new IPS LCD for $200. Other than that there is the bazillion TN panels out there for less than $200.