What is the cost of 4 pounds of butter if 2 kilograms cost $3.20

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.