There are months when I spend nothing at all, and other months that I'll spend 150-400. Most of the time I shop only on sales. I LOVE Connecting Threads for their fabrics, and I do most of my shopping online since my LQS has recently raised their prices to no cheaper than $13 a yard and more if it's batik. I can't afford it. I'm a teacher and don't make that much!
That being said, I do have a rather large stash. I've been sewing ever since I can remember (and I'm almost 28 now), and lots of people have given me fabric, or I've thrifted it from used clothing. In my entire stash I don't think that there's even a FQ of fabric that I've paid full price for. My grandmother also passed her stash on to me when she could no longer see well enough, and so did my husbands grandmother. When I do buy for a project, if a pattern calls for 3 yards, I'll typically buy 4 or so to allow for human error in cutting/sewing/seam ripping. The leftover all goes into the stash. It builds slowly and over time.
The important thing is to remember that it's not a race to get a huge stash and to BE HAPPY with what you have.