LG IPS236V-PN would be a great solution for you!!! I'm currently using an early 2011 15" MacBook Pro and experienced a similar issue when shopping for an extended display. I picked up two of these monitors, which you will often find on sale, if your a bargin hunter..
I normally don't stray away from the Apple brand, condidering everthing I own is mac!! But I found that there monitors are over priced and lack many features that are found in less expensive models, like LG, Dell, Samsung Etc.
LG IPS236V-PN is @ an affordable price range!! Has amazing graphic's!! & Perform's well for less than half the price!!
Hope this helps...
Graphic Designer & Photographer