I checked a few US catalogs, and I didn't find a similar price differential. 35mm professional color film, both negative and transparency, was about $1.50 more than 120. 220 was more than twice the price of 120, however, There could be several reasons for the difference in price. What the market will bear seems most likely. Inventory costs (related to demand) are another possibility. 35mm is probably a bit more expensive to make and market, but not that much.