In the spirit of fair trading consider this:
The everest game takes longer to play than conquer the sea, it also requires a download to your computer. As of yesterday the everest pin could be sold for 50 credits, making each code worth that. The kelloggs codes are worth 20 credits if you sell the room, skeleton, box and table. I would say that if you are serious you should be offering at least 3 kelloggs to every 1 everest to make it a fair trade. Thats assuming also that someone is collecting codes to redeem and sell items for credits.