pigments usually last a VERY long time, an eyeshadow is 1.5 g (from mac) and i basically use it 3-4 times a week for the past year and it's gonna take forever to finish the pan. so if a pigment is 5 g, i think it should last you quite a long time down the road.
personally i wouldn't buy a backup unless its LE and you really like the texture (considering that even with repromotion that the texture might change)