That is extremely flawed logic. You picked an arbitrary example to make your point, but your point requires excluding other possible uses of the Nth dollar to make any sense.

"Oh, with 15k I could buy a compact car that I wouldn't like much, but with 30k I could buy an entry level sports car that I would enjoy a lot. So the second 15k has greater utility than the first 15k."

Does this make sense? Of course not. The first 15k can buy you reliable transportation which is more valuable than the fun from a sports car. 15k can also pay for a year of housing and food in many places, which has way greater utility than the ability to drive like a jackass on the highway.

With your N dollars, you might not enjoy the popcorn without butter, but you could also buy something else to eat, or some fuel, or any number of other things that have more utility than butter on your popcorn.

Every dollar adds additional things that you could purchase. But each dollar comes with declining utility. First you get necessities, then wants, then luxuries. If you have insufficient dollars to buy popcorn with butter, you probably need to spend your money on something now useful than popcorn anyway. Popcorn is a want and you probably haven't met your necessities.