I mean, $12/yr is not a lot, but if the platform is geared for long term investing and not trading, I'd imagine that in any given month 95%+ of accounts are doing nothing but sitting static and handing over $1.
How big is the addressable market here?
Marketing and operations account for a lot!
But that will not pay a modest developer salary. The corporate side of FICA is 7.62%. Also add in health insurance, a retirement plan, office space, a computer for the developer ... and that's just the human side. There's all the corporate costs (e.g., servers) and regulatory filings.
The only play I can see with something like this is to ride it out and hope some established player buys you out for the tech.
I picked a number on the lower side to be generous. Whether this pays for one developer, half a developer, or a quarter of a developer doesn't fundamentally change the big problem: you need a lot of users to cover even a small team and the total addressable market here is small.