"if...were" means nothing today, because woulda, coulda, shoulda. Today white is cheaper, so the calculation stands as it is, and makes for a very interesting argument in today's flood of influencer diets/fads.
It's very unlikely to matter economically for you or me or the op that whiter flour is cheaper. Bringing it up suggests it matters in some other way - like we have an obligation to buy the cheaper option because that has least impact on the environment maybe? But that does not follow at all. I don't think we should award points for originality either.