I've noticed when I'm bumming around my LFSs that 5g tanks cost $20 while 10g tanks cost $15.
It just struck me as odd that the smaller tank would be more expensive. It's the same at all 3 shops I frequent (2 are big box and 1 is a speciality store). All three stores stock Aquarion and/or Marinland brand tanks.
Well, there is some fixed cost associated with manufacturing an aquarium... so it would make sense that the larger a tank gets, the less you pay per gallon. This starts to break down when you get really large as shipping very large tanks is much more expensive than shipping smaller tanks.
It might have something to do with market forces as well - 5 gallon tanks are very popular, especially with beginners (since most new folks don't understand that smaller tanks are actually more difficult to maintain than larger tanks). Perhaps the market has just found that they can charge more for 5 gallon tanks, and people will pay for it.
I think, because most 5g tanks are bow fronts, it might cost more to manufacture them. So jacking up the price is the only way to make $$. Not only for the retailer, but the original company too. Just giving my 2 cents.