No, no, no... the best deal are definitely to be had at the Chinese supermarkets as farrowandballs suggested.
I got 5kg from Summer Naturals a few years ago and I still have loads. However, when I was in Chinese supermarket a couple of weeks ago (where I bought really cheap 5L bottles of white vinegar), I noticed they were also selling 3kg packs of sodium bicarbonate really cheaply. I was nearly tempted to get some more, even though I probably still have 4.5kg left.
OP, if I were you, I would stock up the next time I visit my parents.