Cheat code: (Works on some machines only.) When you a bottle, block em from coming out by putting your hand in as far as possible, then the bottle will be left at the sliding thingy at the bottom. When the machine detects that the bottle isn't out, it refunds you. Do this till you have 2 - 4 bottle lined up at the slide (depending on the machine size), and you can't stack bottle on each other, so buy bottles that are placed far part so they line up nicely on the sliding thing. Then purchase the last one (which is just enough to line em all up) then take em out in quick succession. Saves a hell lot of money.

TL; DR? Then good luck getting em for cheaper.