Whey isolate is expensive long term wise imho, concentrates/blend is typically only 2-4 grams of protein less than isolates (per scoop) and they usually cost way cheaper. Since you're looking for post-workout shake only, concentrate/blend/isolate is the best option to take within an hour of your workout.
This guide should help (a good read):
http://www.bodybuilding.com/store/opt/protein_guide.pdf