It's almost impossible to figure out how much fabric you would need without more info. What were the dimensions of your daughter's quilt? Are you aiming for the same size? Or if you could provide the number of shirts you are planning on using, the size you are planning on cutting the shirts and the approximate width of the sashing, we could probably come up with a guestimate. If cost is an issue, you can always use a thrift store cotton sheet for the backing which would cut down on the amount of fabric needed. Maybe your friend has a sheet you could use.