The 12 oz pop at $0.50 is 2.58 more times expensive than the 2L pop at $1.29.
You could also look at is this way; which one is the better deal? how much money per oz. You pay 0.04166 per oz if you got the 12 oz pop. You pay 0.01907 per oz if you got the 2L pop. The 12 oz pop is 2.18 times more expensive per oz than the 2 L pop.