If you are truly interested in price for the long haul, you might consider DIY. Just like hardware, you can get out of control buying everything under the sun. But, with a little discipline, you can find yourself making great
juice for substantially less than you'll get from vendors.
There is of course some upfront cost: glassware, pipettes, and product to experiment with, but if you read a bit of the DIY forum, take it slow and start with a simple "I just want to make a nice blueberry", you'll do well.
I was going to lay out the math, but suffice to say even using more expensive flavorings you're going to easily be able to make some great juices for <$.20/ml.