Hmmmm... strangely enough, the 5g hex we looked at earlier today was $5 more online than it was in the store. (and actually it was $5 cheaper at Walmart.) Petco had the same tank for $5 more than Petsmart.
Sometimes I get so lost with prices. Fish prices are even more of a mystery to me. Sometimes the LPS has better, bigger fish than the chains for less money. Sometimes the chains have vastly different prices for the same fish (up to $2 per fish).
I guess you just have to shop around