EDIT: Please read my Nearly 3 Years Later: 3 Broken Bones & 200+ Pounds - The Long-Term Effects of Juice Fasting update before considering juice fasting. I hope that my experience will provide you with real-world information about long-term juicing.
The overall cost of juice fasting depends on several variables including when you fast (fruits and vegetables are more expensive during the winter months); where you shop; and whether you buy organic or non-organic produce.
On average, I spend between $150 - $200 a week. This includes the occasional kombucha tea (yes... I haven't given those up) and Gerolsteiner mineral waters. This figure isn't written in stone; yours may be more or less depending on how much you drink.
Of course I try to buy only organic produce, and I tend to shop at our local grocery store chain. I have completed fasts using Whole Foods produce, but I have noticed that I can purchase just as nice fruits and veggies at a lower cost elsewhere.
Hopefully this will give you an idea of how much it may cost. Remember, there's a chance that it's the choice between a bump up in your grocery bill or potential medication costs for weight-related ailments.