I know that day packs range from $30 to $300, depending on all of the features. I like to check our REI when they have outlet sales (or whenever I can make it up in the Charlotte area) and I have gotten some nice packs. I have an older military rucksack too. Some of them have water bladders and others don't. I have noticed that if the straps aren't padded enough and I'm carrying water bottles, other heavy items, I feel some real shoulder strain as the day goes on. This is the case with my Flash 18 pack.
I struggle with breathable material vs waterproof. Smells, spills, etc all seem to happen during the most inconvenient times.