Wondering how much luster dust I will need If I am using it to airbrush 150 cupcakes gold. Planning on tinting the butter cream a light brownish first. 

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one of these would do it -- there's a smaller size that would also be enough but this size is more economical

http://www.globalsugarart.com/classic-gold-shimmer-premium-airbrush-food-color-ounces-by-chef-alan-tetreault-p-32451.html

and of course you would line them all up and spray them all at once in a group and just turn the group of cupcakes around and around to get all sides -- don't spray them individually -- i would recommend going with a brighter color underneath like a golden yellow -- 

while they are in this grouping -- protect the perimeter cupcake wrappers with paper towels or parchment to deflect the spray -- you don't want the guests to get gold paint on their fingers at the event --

best to you