You're saying that if an employer expected you to pay for the flight for an interview, that would be a red flag.

But then you say that as an interviewer, you would be willing to pay for the flight for an interview (if you thought it would be reasonable ROI).

The situation where you would be willing to pay for the flight implies that the employer would not pay for the flight (or else why would you pay for the flight?). So according to your own logic, that would raise a red flag (because the employer won't pay for the flight and expects you to). Then why would you be willing to pay for the flight to interview at an employer that is raising a red flag for you? Makes no sense at all.