Assuming you're talking about 10 users, 50/usd/year/head does seem a bit steep. It's what you'd pay fastmail or user for a custom domain.

I'm a bit surprised fastmail doesn't appear to have a group account that's at least a little cheaper.

You can get less (in way of email service) for much less from something like:

https://www.hetzner.com/webhosting/level-1

That's less than 30/year for 100 accounts, custom domain, imap/pop/smtp (just 5gb storage shared though).

Now, I'm happy with hetzner as a dedicated hw host - but there's a bit of a bad neighbour effect - and you obviously get less for 30/year than for 500/year...

But seems strange that fastmail doesn't have something in between for family accounts. Maybe they do if you shoot them a mail?