Fresh popcorn serving size suggestion
Suggestion:


Make a batch of popcorn, weighing all ingredients used.
Fill one bag.
Weigh the bag.
Calculate price per serving and total amount of ingredients required based on the weight of your initial batch.


In short, decide what kind of bags you have / want to use and calculate from there.  

(Hint: We often instinctively think of popcorn as a volume, not as a weight based quantity.)

If you don't have the bags yet, meassure what you consider a one-person serving. I suggest making servings of plain popcorn larger and those with lots of sugar or similar smaller - e.g. 2 generous handfulls for caramel corn vs. 4 for plain salted.
Put it in a meassuring jug, read the volume scale, buy matching bags. If you need to calculate ingredients, weigh your serving and proceed as above.