if money is an issue, i don't buy the fabric line for the backing. i find a cheaper co-ordinate, like muslin or a solid and use that. i use the real money for the front only. what size do you usually make? that determines the yardage. for a king, i buy 5 of the major one and two each of the others. that gives me leeway to do whatever, even though there's lots left over for stash. a king size quilt needs almost ten yards total for the backing, so that should give you an idea of what you need. remember the seam allowances and bindings.