Estimate popcorn cost for movie nights.
Popcorn from kernels is priced by weight, so the cost per night is grams per person times headcount at the per-kilogram price, plus the butter and seasoning. The numbers are small and that is the point: 35 grams a person is a generous bowl and costs a few pence, against a couple of pounds for a microwave bag or many times that at a cinema. The popped volume is included because 140 grams of kernels fills roughly three litres, which is more than most people expect from a small measure.
Kernel cost
Kernel cost = people x grams each x price per kg / 1000
Per night and volume
Per night = kernel cost + toppings; popped volume = kilograms of kernels x 20 litres
Substantially. Loose kernels typically cost a tenth to a twentieth of the equivalent weight in microwave bags, and the difference is almost entirely packaging and flavouring you can add yourself for pennies.
Twenty-five to thirty grams of kernels makes a decent individual bowl; thirty-five is generous. A large sharing bowl for four is around 120 to 140 grams, which pops to roughly three litres.