I dont, this is something I enjoy so I don't put a $ amount on it. However, I don't go and spend hundreds of dollars every time I am at the LQS or JA. I buy what I need when I need it. The only other hobby I have is photography and I already have everything for that.
My husband's hobby is photography and he is always spending more/buying some new gizmo - Not to mention his camera cost five times what my new sewing machine cost! So I don't go overboard on collecting fabric, but I don't feel guilty when I buy some either. Usually just buy what I need for a planned project.