My sister went out and bought a 5 gallon hex "aquarium" at Wal-Mart, cost her like $40. I told her she could have bought a 20 gallon and saved some money, since I could have given her the filter, heater, gravel, and light. But she wanted the cool looking plastic hex.
It is definitely overpriced! The market for the high priced, small tanks is on a desk that's in an office. People don't want an eyesore. They want there "minii" aquarium to look cool!
There is a market for them otherwise you wouldn't see them for sale.