I'll let others weigh in on the backpack options. I just wanted to chime in that you might also want to consider a checkpoint friendly laptop sleeve which then lets you select from many more backpacks. I've used the
http://www.sfbags.com/products/sleevecases/sleevecases.php for several years for this.
Waterfield is bulletproof, but not exactly inexpensive. Since you are trying to keep to a budget, I'd suggest looking for a less expensive option which leaves more money available for the backpack.