Roger, from what I see the prices must be figure by a formula more complex than just surface area. They must be putting a value on each format based on volume, cost of production including confectioning etc.. I would guess that the cost is offset by a higher cost for the format and adjusted prices for the more popular formats to cover the cost.
I'd be interested in seeing Pan F+ in sheet film sizes but the capability of production doesn't exist.
This did cross my mind but the 4x10 is over three times the surface area and yet the price is within a dollar or two. Maybe I'll PM Simon about this as it is really bothering me.