The two Glacier Water vending machines at Bell Fuel & Food, on the corner of Cave Creek and Bell Roads, have the cheapest price in town (according to Derek, the service technician who inspects 150 machines in north Phoenix). Only 15 cents a gallon, as of summer, 2010.
As an added bonus, pull up to any Glacier machine while the technician is servicing it and it's the company's policy to give customers free water -- even if you've got four or five 5-gallon jugs in your trunk! Thanks, Derek!