small, medium or large eggs
It depends on the time of year (and, of course, sales). 

One book I read said that since many chicken farmers raise extra birds to be slaughtered around holidays (Thanksgiving, Christmas, and Easter are the big ones), often they'll have shortages of smaller eggs (which often come from smaller chickens) around those times. Since USDA regulations give a minimum weight for egg types, but not a maximum, the chicken farmers tend to fill out orders with larger eggs. You can always weigh the eggs in a pack of small or medium eggs to see if they are actually that size or larger. 

According to the USDA standards (see page 29 of the document), the minimum weights for a dozen eggs are: 


Jumbo: 30 oz. 
Extra Large:  27 oz.
Large: 24 oz.
Medium: 21 oz.
Small: 18 oz.
Peewee: 15 oz. 


(It's a bit more complicated than that, since they actually weigh them by a lot case, which is 30 dozen eggs. But that's close enough for most purposes.) 

You can actually figure out per egg cost at home based on circulars, and then weigh at the supermarket to see if the eggs are truly what they're labeled or if they're running big in that lot.