In this modern space age in which we live, we'll need to be careful.
2 kilograms weighs 4.409 pounds on Earth. If that much costs $3.20, then the cost for 4 pounds is
(4 / 4.09) x (3.20) = $2.90 .
But what if you grow up to live or work on the Moon ?
2 kilograms weighs only 0.73 pounds on the Moon. If that much costs $3.20, then the cost for 4 pounds up there is
(4 / 0.73) x (3.20) = $17.54 .
The thing to remember is that if you make frequent trips between the Earth and the Moon, make sure you buy stuff by mass, not by weight. The weight changes when you take it somewhere else, and you don't want to get cheated.
