Cheapest place I have found locally is actually Central Market. They got the little can for $12.99 and the bigger one for about $21.99 or so. I think that is a little cheaper than Smoothie Factory, a lot cheaper than Smoothie king and a HELL OF A LOT cheaper than GNC, which sells the little can for $17.99. Thats highway robbery.
Central Market used to sell the small can for $10.99 and the bastards went up.
You might also look at Discount Sports Nutrition's website. The prices on their site are cheaper than you will find even in their stores. I picked up a 5lb jug of Optimum Whey for $22.95+shipping. (BTW, Optimum's Chocolate whey tastes like burnt ass )