I’d agree it’s very close to 0. Closer than on-premise in my experience as support costs are far far less for SaaS vs on-premise.
Some types of SaaS this might vary a bit, like Snappy has some hard costs per email sent/received that do increase with accounts (though not necessarily so much with users in accounts) but even those tend to be pretty minimal and as long as you’re not doing free accounts are easily covered by even the smallest amount of revenue.