I generally buy batting by the bolt. Finding the best deal can be very confusing, because of the different sizes, discounts, sales, etc. Recently I made a spreadsheet of various prices that I could find for different battings made by the Warm Co. (Warm and Natural, Warm and Bright, Warm Blend, etc.) at JoAnn's and other stores. I normalized all the prices so that I was comparing apples to apples as far as a square yard of batting went. What really surprised me was that the 45" W&N at JoAnn's was a **LOT** cheaper than the 90", even after you factored in that it was only half as wide. That means that if you wind up piecing your batting anyway, you're better off buying the 45". At that time, the 90" roll (40 yards) was $429.99 and the 45" roll (also 40 yards) was $159.99. That means for the same amount of batting you could buy 2 rolls of 45" for $319.98 (instead of one roll of 90" for $429.99) and save $110.01. (Of course I would be using a coupon on either deal, or waiting for it to be on sale, but you get the idea.)
Sometimes it's worth it to me not to have to piece the batting. But if I'm making a larger quilt I have to piece it anyway, and if I'm making a small one I try to use up the scraps, so I almost always wind up piecing it to some extent. I think if you can find a sale on the exact size batting that you need, that is probably the most economical way to go, but how often is the exact size that you need in a package? That means you wind up with leftovers, and I find I can use the leftovers from the roll more efficiently than the leftovers from a packaged batt.