One thing I'm still not sold on is that interns are a net loss. They would often get to do work that was novel to them and tedious to your full-time developers. Work that was nice-to-have but no-one-wants-to-do. It's hard to put a price tag on it. Especially with externalities that come with that kind of work; bad ones like killing employee morale for whoever has to do it and good ones like increasing the team's productivity. Interns, in my experience, are not prone to the former yet still contribute to the latter. Examples of such work are things like updating tests and documentation or tackling some not-so-urgent tech debt.
Assuming that they still create tech debt of their own because they probably were (and we all are, aren't we?) my experience has been positive.
But that's for a Senior or Principle dev - for intermediary devs I wouldn't expect positive gain for the company for a good two months and junior devs tend to be more expensive with that training phase lasting for something like six weeks.
Interns are a loss for companies - but please don't stop hiring them, you'll get some great employees out of the process and you're also investing in developing better communication and mentorship skills for your long term employees. As someone working at a company that was teensy tiny when I started this latter point is quite important - I actively invest in my skills to keep up with the curve and make sure I'm still providing as much value as is reasonable for my position. If I had been locked away to dev code in isolation when I was hired I'd be super rusty at a lot of skills that provide my employer real value.