It really isn't cheaper if you think about it unless you place no value on your own time. If you factor in grain/extract, hops, fuel for the heating, gas for kegging, equiptment, cleaners, sanatizers and most importantly time you don't save much money.
With that said, to me it is a hobby meant to take time and I enjoy every aspect of it (even cleaning bottles and actually bottling). I look at it like this...without factoring time into the equation, I can brew 5 gallon batches of great beer for the price of PBR, feel productive and accomplished while doing it and most importantly have a ton of fun every step along the way