But the cost of Amazon Prime have to be factored in as well
Only if you weren't already paying for Prime. If you were, then it's irrelevant as this usage adds no marginal cost.
No, for anyone considering doing this it does not have to be factored in as it’s a sunk cost.
Only if you weren't already paying for Prime. If you were, then it's irrelevant as this usage adds no marginal cost.