The kind of shot your looking for makes a big difference. Steel or Lead shot? Are you hunting upland game bird or waterfowl? If its waterfowl, you cannot use lead shot so that pushes the price up. The last box of steel shot I picked up (6 Shot) was $12.99 per box (Federal Speed-Shok). Bismuth Shot goes even higher at around $18 per box.
for Lead shot if your hunting upland game birds, it can be had for as low as $7.99 per box. I would avoid the Winchester Universal though. I don't know about other peoples experience with it but it always seems to jam up in my pump action every 4th or 5th shot, and its extremely dirty. It takes me 3 times as long to clean my shotgun after i use it compared to other shells.
If your not hunting and just using it to bust clays, then the cheapest would be Federal Top Gun at $6.99 per box if you buy it by the flat at Wholesale Sports, $69.99 per flat.