If you have a developer on your team, Twilio is pretty simple to set up and extremely cheap (fractions of a cent per text message).

As your system gets more complex, though, a custom solution may be a bigger hassle than it's worth. It depends on how technical the people on your team are, I suppose.

I've built simple systems like what you're after, and once the upfront cost is taken care of, the cost of sending messages themselves is negligible. Provided the system doesn't need to change or add functionality frequently, of course.

If a custom solution is something you're interested in, I'd be happy to walk through the implementation details with you or your team. Good luck!