This is the kind of scientific investigation that organizations like Consumer Reports typically do to rate different brands of things. Consumer reports comes up with a rating system that typically involves several individual factors, price being one of them, and then they come up with some overall rating that combines all of the factors to provide "Consumer Reports best buy recommendations".
If Consumer Reports would do something like this on popcorn, they would likely be acquiring multiple samples of each brand from different sources, recording the price they paid for each sample. They would definitely use the same equipment to pop each batch and either follow the instructions provided by each brand and/or come up with a consistent method and use it for all brands. They would then come up with some consistent evaluation criterion.
If you want to be scientifically fair, you need to acquire and pop multiple samples of each brand from different stores to insure that your results aren't influenced by a single sample of a brand that might not have been transported and stored appropriately. You might want to pick three brands that are all carried a three different high volume grocery stores in your area.
You should do some research into different popcorn varieties. For example, http://www.yoderpopcorn.com/
grows and sells a wide variety of genitically different popcorn (I try and stop by every time I'm traveling in that area). If you were comparing their Lady Finger popcorn (miniture kernels) to their standard white and yellow varieties, there would be a significant difference in the volume of popcorn per kernel (standard varieties have far more volume than the Lady Finger); however, if you compared the popped volume to the amount of hull, the Lady Finger would come out ahead. I'm not recommending that you actually use these specialty varieties in your experiment, I'm just pointing out that you need to consider if there a significant genetic difference in the brands you actually use and determine appropriate comparisons for the brands/varieties you end up testing.