Because the time cost of getting out your credit card, risk of unwanted charges, overhead of managing a subscription, a new account per password, far outweighs

Because the time cost of getting out your credit card, risk of unwanted charges, overhead of managing a subscription, a new account per password, far outweighs 2.5$ The difference between k+2.5 and k+10 is very small. But you quadruple the income to the business, and so the value of the product (more resources can be invested) K can be labelled as transaction costs, price must be higher than tx costs.