Can of Coke about 25-30p from Makro (or even my local pound shop, though it's in Turkish), and I'm sure EC can get a better deal. Or if they're bothering to serve OJ in glasses, get the 3l bottles of Coke in at a ridiculously cheap price and serve that in glasses - or does it look 'common' pouring from a large plastic bottle? Airlines have no qualms about it.
While I'm sure some people would be snobbish about it, I regularly buy own-brand cola (and lemonade) from supermarkets. Yes, the taste is different to Pepsi or Coke and I am not going to pretend otherwise, but you could still offer fizzy drinks that taste perfectly good to people with little cost. Most likely cheaper than OJ. Cola is probably the one drink people have most 'opinion' on, but less so on a lemonade or fizzy orange.
However, failing that, serving carbonated drinks from bottles instead of giving away expensive cans (which also results in more waste to manage) makes sense. It does go flat once opened, but not that quickly, so the wastage there would be (should be) chucking away a bottle if it has been opened for longer than xx hours - certainly not keeping anything overnight in a fridge.
What a shame I've not seen Virgin cola for years - I'd love to see EC offering that.
