For anyone doing this as a hobby sure, but if this is your business then it’s nuts not to just pay the cheap toll.
So it's not 120$/year - it's 120$/year/developer.
And setting it up for yourself - I've found multiple instances where having a configurable nginx reverse proxy on a public server saved me a bunch of time, it's much more flexible than ngrok.
If I have 100 devs that's $12,000/year (ignoring any bulk discount), right? Are you saying that in a company with 100 developers they cannot spare enough time to run their own server cheaper than that?
Additionally, while the amount is 'cheap' in the isolated sense, when you start adding all those 'cheap' services together they quickly add up and suddenly your per-dev costs start getting out of control. In all cases you should consider the value of the service being provided. While I love ngrok, the functionality can easily be replaced for most use cases and move you from a per-dev cost to a relatively (yeah, large numbers of users will cause server scaling/cost changes) fixed cost instead.
Edit: Just to add on, $10/month is more than I pay for Gsuite or Jira on a per dev basis. The Ngrok pricing is drastically off base IMHO.
Nginx or Caddy aren't all that hard to set up, for example: https://blog.kronis.dev/tutorials/how-to-use-nginx-to-proxy-...
Actually with WireGuard, OpenVPN and the like, you can do some pretty interesting (and sometimes problematic) things as well, so there's definitely a lot of flexibility with a few footguns here and there: https://blog.kronis.dev/tutorials/how-to-publicly-access-you...
Disclaimer: everyone's setup has their own requirements and whatnot. If using external services works for you, that's great! Also, that second article of mine may serve as an example of that NOT to do (you typically would only want to selectively forward ports, rather than forwarding all of the traffic like i did).