Dark souls 1 sold 3 million copies on steam. (steamspy data) The cheapest I've seen it on sale in my region was £10. (steamprices data)

They definitely made their money back and then some, as they then went on to port DS2 and 3 to PC.

Port costs are quite variable so you can't easily put a number on it.

It depends on what platform you are porting to and from. PS3 to PC is much harder than PS4 to PC because PS3 is more architecturally different than PC.
Persona 4 came out on PS4, so it shouldn't be that hard.
It also depends on the competency of your developers in making PC games, PC is a harder platform to test and develop for than consoles because of hardware variations, crappy programmers who only have to get used to one hardware setup, are often not good at porting to PC.
If you have a bunch of programmers who are actually not that technically competent, but just scrape by good enough to make a console game, you may need to spend money on training, hiring others, or outsourcing a port to a third party.
And of course it depends on your production process. For example, if you are porting to PC, you may find that your textures for PS4 look crap on 4k resolution, so you need higher definition assets. If you were planning a PC release from when you started development, you make your masters in a high res and downsample them to levels appropriate for each platform. If you just make textures for fixed hardware though, maybe now you have to recreate or upscale art assets. This extends to the whole development process. If you develop in mind for platform portability, your code and structure of your software will be ready to do that. If you don't it may need more changes.

Japanese companies are often very top-down, bureaucratic, ego-seniority fests, (I lived in Tokyo for two years) which often means they have piss poor organisation and planning and reactions to changes or doing things in new ways. As such, it may be way more expensive for Atlus to Port Persona 5, than it would be for a typical western developer to port their similarly sized rpg. Many decision makers at Japanese firms are very old, and very out of touch, and very traditionally minded, and have huge egos, plenty of young japanese people bemoan this (politely and respectfully and subtly) all the time.

But they are still very likely to make a healthy profit off it given the popularity of the title and demand, if they do it soon enough and don't badly fuck up the port. They would have made a lot more profit, and been able to port it for a lot less cost by aiming to be doing it at launch though.