Wicked Lady has the right idea, sort of.
I use cost-per-wear with clothes figuring if I wear a $200 cashmere tee a thousand times, it only costs me 20c every time i wear it and it will be as good as new on the thousandth wear if i look after it. but if i buy a nasty $10 shirt and wear it a few times before it gets stretched it costs, like, 50c per wear and sucks.
With food I buy the homebrand shit cos it tastes better (honest) and cosmetics it tends to depend.
cf.
generic vs' name brand