You might want to consider a normal backpack, and then something like a basket or pannier to put it in. Should come out a bit cheaper than the Bug.

I have a laptop backpack I really like, but I don't like riding with it on my back. In the next few days I have been planning to get a rear rack with a basket or milk crate on it to drop the backpack into when I ride. The basket will also be useful on days when I don't have a backpack with me, and want to stop at the shop on my way home or something like that.