The kilogram isn't a constant, just a standard measure. It's exactly the same as measuring distance; you can do it in feet & inches, miles, nautical miles, metres or kilometres.

They're all just different sized units to measure with. The reason that the kilogram has become a 'standard' is that it's decimal based, which makes it easy for us to do maths with, and that its size works nicely with the other SI units. For example, if 1kg of flour costs 1$, then one tenth of a kg of flour would cost $0.1, but if you used the old UKP Sterling and 2lb of flour cost £1 then one tenth of 2lb is 3 ounces, 87.5 grains, and it would cost you two shillings, or 24 pence.