I would argue that only covers half the equation. Prices should be congruent with increase in value which in itself is dependent on what is meaningful to the us

I would argue that only covers half the equation. Prices should be congruent with increase in value which in itself is dependent on what is meaningful to the user. Mailchimp is pricing for increase in value as well as users find more meaning in having larger mailing lists (which also happens to increase costs). Optimising purely for costs is not always the most effective and misses vital opportunities.