The most inexpensive batting is poly. Cotton prices have skyrocketed in the past few years. Hobbs Polydown is a fine poly batting. I use it often and it is good quality and doesn't bunch. But it is slippery so you need to baste your quilt sandwich well.
www.connectingthreads.com often has sales on batting at 30% off with free shipping at $50. In fact they are having one right now. Many stock up during this sale as the prices can't be beat.