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Pay-as-you-go can quickly become complex unless you have a very simple, one-dimensional utility service without any premium features. Otherwise your pricing will look like that of AWS (i.e. may be cheaper, but less predictable). Tiered pricing is simpler, lets you define a minimum monthly cashflow from each paying customer, and most importantly lets you discriminate e.g. enterprise customers by offering features only they would want at a much higher price.