As someone that has purchased $1/gallon tanks, I have noticed the glass is thinner leading to a bow in the 29 gallons. I am not sure if the 40/50 breeders sold during these sales have a center brace, but I image they would be worse. So if your breeder tanks came with thicker glass, center brace and silicone options then I would pay more. Your 70-80 for 50 gallon seems fair.