Monzo's core banking product (current account)

Yes relatively low, compared to some of the products that banks offer (& Monzo won’t).
Monzo’s first significant revenue stream will come from overdrafts. Obviously most banks couldn’t earn enough from overdrafts to cover their costs either but crucially, Monzo doesn’t have a network of physical branches to pay for or (to a lesser extent), a legacy technology stack to maintain.

To put this in context, Tom’s said that Monzo is currently losing £40 per customer, per year (there’s 80k users now…) & will lose £20 per customer py once the current accounts launch. Although I’m not sure that those figures include Monzo’s overheads - which are relatively small, as it’s still a ‘scrappy’ startup.

Overdrafts are just the first product though, Tom said at a presentation which he gave a couple of days ago that eventually, he expects Monzo’s online marketplace for additional products from 3rd parties (which will earn Monzo referral fees), to be a much bigger source of revenue than the services that Monzo supplies directly. And obviously a marketplace costs a lot less money to maintain than providing the services directly, while also keeping the size of Monzo’s balance sheet small.