I hadn't considered that it might be the retailers, but that may be the case. The price of 120 film seems to have stayed reasonably consistent over the last couple of years. It has gone up, but seems to have gone up at similar rates everywhere. 135 film seems to fluctuate significantly. I saw Provia 400x for almost £13 recently and it is now back down to £10, from the same retailer.
I still think the price discrepancy can't be explained by the material cost as Fuji can make cheap 135 slide film to sell on to AgfaPhoto.