J.B.
I would think that for a juice fast, organic produce is a requirement. Otherwise, you're defeating the purpose of removing toxins by then filling back up with more toxins, kwim? I know it's expensive but if you're going to put the time and effort into this, you want to do it right and have it actually work. No point in suffering through a juice fast if you're not going to accomplish your goals, right?
As for pricing...I think it's too highly local for me to give you any good info. Around here, there is a chain called Market Basket that is vastly less expensive than other grocery stores but I think they're just a New England thing.