I give away the majority of the quilts I make, to friends and family, and only recently a couple for charity. I like to do scrap quilts, though, and have only once bought the entire amount of everything for a quilt at one time. That was for my grandmother, and I estimate that it cost around $50 for everything, maybe less. It was a large lap size. Most of the fabric came from Joanne fabric.
Usually I only buy one or two fabrics just for a particular quilt, and the rest is scraps I already have, some of which are from other quilting projects, some from clothing projects, and some that others have given me. So it's hard to estimate cost. I'd say I've got around $100 in batting, backing, and sashing for the queen size I'm working on, more in the fabrics used for the blocks, but they're scrappy, and I have no idea how to estimate that.
I know I've made at least two quilts that were entirely from scraps or yardage that was given to me (and even batting) and the biggest expense was the fabric for binding.
Now that I quilt, I try not to make clothes out of fabrics that I wouldn't want to quilt with :)
I also tend to buy most things on sale or from remnant tables, thrift stores, etc, so that keeps my fabric cost down.