Possible budget lightweight backpack: 6oz 28l IKEA KNALLA backpack costs $4.
Black: http://www.ikea.com/us/en/catalog/products/90330483/
Red: http://www.ikea.com/us/en/catalog/products/40330485/
If you’ve ever tried it – what did you think? I’m guessing at that price it would not be very good. For example, I don’t see any strap padding.

