The first thing you have to realize about fountain soda is that its price is almost entirely profit. The syrup+co2+water+cup+straw cost the business something like $0.10, so whether they sell it at $1 or $3 is a profit margin of either 90% or 97%, both stupidly high in the business world. A lot of fountain soda is sold in places without competition like restaurants and movie theaters -- people go there for something else, and once there, are willing to massively overpay for soda.

On the other hand, bottles of soda from a vending machine are kind of logistically painful. It's not the soda that costs money, it's the putting it in a bottle and putting the bottle in a case then putting the case in a truck and driving it somewhere and having someone put it in the vending machine then paying for electricity to keep the vending machine cold 24/7...

The bottom line is, you can't really expect consistent $/oz pricing on things like soda, because the actual $/oz for the soda is always such a tiny fraction of the sale price.