Almost any of the whey powder's are good, but one of the best things for a post workout drink is low fat chocolate milk. I take a quart with me every day to drink after my workout. The cost is really nice too, cheaper than a large coffee.
Just started using a protein powder to keep me from falling off the scale while doing cardio this month. Loving the ON Double Rich Chocolate mixed with skim milk. Just picked up 10lbs for $85. Should last me the bulk of this year.
Ten pounds would last me about two months, two and a half at most.