That may not be any cost improvement. 3 @ $4.75 = $14.25. 3 @ $10 with $2 refills = $14.00. Can you really drink more than 3 souvenir sodas a game ?
If we haven't scared NATSCSMGR off, here's a serious suggestion: Can you offer a large souvenir cup or sports bottle for fountain drinks at a price of say $10 or $15 (or whatever) then offer discounted refills (at a price of something like $2) all season long? Amusement parks like Kings Dominion and Busch Gardens do this. It'd be a nice gesture to season ticket holders and others who attend a majority of the games. I'm a big soda drinker, but I balk at paying $4.75 or more for a drink so I'd definitely go in for something like this.