We don't differentiate on price :)
Are you able to disclose whether the $50 a month is a result of nab's involvement somewhere in your end-to-end setup? To me, that's the most unfair part of payments in Australia. I can understand charging if an account is dormant but not just because everyone else is doing it. That's the problem with banks today- they all just copy each other.
Whether it's because they too rely on nab (An Australian bank) to provide the multi-currency account (Also used by Braintree) or because they aren't looking to differentiate on price I am not sure.