I know they don't take 30%, but I can't figure out what the cost is. According to Apple, it's free, but does the seller generally have to pay more in CC fees to get Apple Pay support?
Quick 101 on how card fees work generally:
Card issuer banks (Citibank, Chase, Bank of America, etc.) contract with a card network (VISA, MC, etc.) and earn a standard fee schedule called Interchange (it's public, you can look it up, typically 1.x-2.x% depending on the card type). This is the bulk of where the card fees go, and often funds card rewards and benefits.
Meanwhile merchants contract with processors (e.g. Chase Paymentech, Heartland, First Data, Square, etc. etc.) who interface with all the card networks, marking up those interchange fees with their own margins. The processors may set up merchants with equipment, or maybe a third party POS vendor does that. But most fundamentally processors are responsible for any merchant-related fraud on the network.
That is why processor markups and merchant vetting procedures can vary -- and why Apple would never take the place of the processor. It's merchant-specific work and involves financially vouching for them. The less vetting the processor does (Square, Stripe), the higher the markup. The more vetting, the close to interchange the fee is going to be.
[1] https://www.digitaltransactions.net/apple-pay-no-charge-for-...