Not to turn this into a paiiers versus backpack thread, but just to throw in my $0.02, I prefer a backpack strapped onto a rear rack for many reasons. I use a pair of surly junk straps ($8 each) to fasten the backpack to the rear rack & it works well for me. The reason I mention it is that you said you're looking to do this inexpensively & if you already have a backpack, this might save you some $. Panniers also have their benefits.