For #9 the 18 ounce packet has the lowest cost per ounce
I know this because for the 7 ounce packet it is $0.37 per ounce, for the 12 ounce packet it is $0.38 per ounce and for the 18 ounce packet it is $0.35 ounce a packet. So the lowest is the 18 ounce packet.
I hope this helped.