Being in the retail business myself (auto parts though) I think I know why this is the case. Supply and demand plays a part as all those new betta owners don't want to put their betta in a 10 gallon tank. So they buy a 2.5g with a glass lid. I'm willing to bet they know this so the jack the price of the 2.5g up. Also, as has been mentioned, the 10g is a bare tank. Most 10g starter kits cost approx $30us at walmart.
The flipside could be they don't sell nearly as many 2.5g or even 5.5g tanks as they do 10g tanks. When bought in bulk, the more you buy, the price per unit drops.
My 5.5 gallon bare tank cost me $12 and change at petsmart, and my 10 gallon bare tank which was supposed to be a sump for my 20H sw project cost me $10.