I find that the cooler bags vary a great deal with their ability to keep ice from melting. I grabbed one from K-mart for $15 and the ice melted pretty fast. Now I'm using one from
http://www.polarbearcoolers.com and it keeps the ice loooong time, so I'm stoked with. I also just noticed that they have bag coolers at Costco for $7. They aren't as thick as my Polar Bear, so I doubt they retain ice for long, but for $7 it seems like it's worth a try!