20oz are more expensive per volume because they sell faster. There's less of a demand for larger sizes typically go flat too fast for people unless they're having a party or something, and even in that case they don't have the convenience of being able to drink from the bottle.
Do you leave your soda uncapped or something? I buy almost exclusively 2 liter bottles and they very rarely go flat on me. I'll drink one over the course of about 4 days.
When I was in college a gas station a block from me would do incredible deals on soda 12 packs abouth once a month. Like buy one get two free. We'd stock a full month's worth every time and basically have a soda mountain for people to rummage through
I've had the best luck with Dollar General for affordable drinks. You have to wait for a decent sale and stock up though, and use coupons when available.
For example: recently for Dr Pepper they had two 12-packs for $14, but you could buy three for $15 and there was a coupon for $2 off when buying three, making it $13 for three 12-packs. So you would pay a dollar less for 36 cans than you would for 24. Comes out to about $0.36 per can.
You know how many smokers have/had their hard line on what price would make them quit smoking? For me (a 52yo woman) that was when pop breached $5.99 a 24 pack case. I would buy it again occasionally if sale prices dipped into that range, but having crossed that line it broke my habit. The thought of $7 for a 12 pack is just painful.