In the UK, I sheet B&W sheet film is a lot cheaper than colour, probably because all the cheap colour film like Superia or Gold, are not available in anything other than 35mm. A quick spot check on AG Photographic tells me 50 sheets of Fompan 100 in 4x5" is 26.90, 10 sheets of Ektar is 30.50. A big difference, and Ektar is the cheapest colour film I know of which is available in formats bigger than 120. I think in 35mm, the situation is reversed, and the cheapest colour film is well under 2, the cheapest B&W I think is only a little under 3.