When you are buying a phone, you also pay price that includes price for OS of the phone, that includes development costs of SDK and development tools. Some of the phones nowadays has a choice to BUY and install different OS, that comes with different stores.
Unlike your assumption, payment processing is completelly different ecosystem and Apple is simply using monopoly on iPhones and not letting other stores on their hardware and software. Alphabet does not produce phones... so IMO it is acting worse, that it makes up these stupid tales. This is one of the reasons why I am switching my phone from Android, because that is MY phone, that I paid for! Besides, Android still snoops your data without your consent and I'm not even paid for that data!
Besides, the only concern about payment processing that YOU should have is not development costs(they can't be that HUGE...), but if your payment processing is safe and protected!!! That is the only costs, that stores should have - to ensure protection of those many many transactions, where maintenance and development costs are miniscule and to be even considered is laughable topic.