Where is that WPX $74.64/ton opex coming from? According to my knowledge, it is $62.28/ton.
You also forgot to add extra $15 million yearly sustaining cost to KRN, which means $7.5/ton, but you added this cost to WPX. By the way, there is no way KRN freight is $9 cheaper then WPX. It should be the same for both companies since they are in the same province and have almost the same distance away from the ports. WPX uses more conservative numbers then KRN.