I don't think reasonable prices are based on what the potential maximum profit per user is. Normally it's based on the expense per user plus a percentage for profit.

Your mistake is assuming that "reasonable" exists as something concretely defined. Reasonable is an opinion.

Reasonable is definitely an opinion, but that doesn't mean it's random.

In legal and economic contexts, reasonable profits are a thing that is routinely calculated. Where industries are more regulated, 'reasonable' profits are often bound by statutory guidelines. In more competitive markets, prices are not fixed in the same way but you can take a look at industry standards to ascertain what might be reasonable.