Penn-Plax Water World Radius Curved Corner Glass 7.5 Gallon Aquarium Kit – $46.19

Best price besides one day in October last year. Normally averages around $60. This is a great price per gallon for an all-in-one, though some people have opted to replace the fitler.
https://smile.amazon.com/gp/product/B0069RTX3I
NOTE: Ships from and sold by Petco, through Amazon marketplace.