Much of it is based on your cage design. I can buy a roll of 1"x2"x36" and a roll of 1/2"x1"x30" and make several cages in banks. I cut the 36" 1x2 in half, so my cages are actually 17" tall. This doubles the amount of wire and lowers my cost. I also build banks of cages. This saves the amount it would take for some of the side walls, plus all of the waste you get with each cut. since the cages are 30" deep, when I cut the wire for the tops, there is an extra strip. I use this to build my hay racks. You also need to compare the wire that you would be purchasing in a roll compared to the wire that would come if you were to purchase a cage.
I do agree that if you are building individual cages from Bass vs. their rolls of wire, the cost is comparable and you don't have to spend a lot of time cutting and you don't have the issue of miscounting and cutting in the wrong spot (which has happened a few times here when I hear, "Mom, mom, mom, mommy, mommaaaaaaa. Whatcha doing????").
I have bought the 1x2 at farm stores here and found that it isn't nearly as nice as that from Bass (haven't tried Klubertanz or any others, as the shipping from Bass is excellent) and it doesn't line up when counting the squares. Measuring is a must, especially if you are building banks.