>>>Honest question: what is his realistic chance to get laid with non-prostitute?
At the time, pretty high. The clubs and bars were very active, and "gaijin-hunter" Japanese women are notoriously easy. If you are halfway-attractive and at least a LITTLE charismatic, you should be able to snag a one-night stand or two, and then a girlfriend in.....6 weeks? That's assuming going out Friday/Saturday every weekend. If you aren't drinking alcohol, it's not that expensive either. If you end up with a relationship that lasts a few months or more, I consider that a pretty good ROI.
>>>How much money would it cost to pay sex worker?
Depending on service (incall/outcall) and location...$100-$300+.
>>>I also suspect that going around bars while not drinking looking for girl to have sex with might involve a lot of rejection and a lot of effort.
It does put you "not on the wavelength" of people when they are drunk, doubly so if they are kinda dumb too. You can overcome these hurdles by being attractive, and knowing the right conversational hooks to say in their native language to build rapport quickly and lazily. Building that skillset does take time, and brings other risks (PUA's call it "calibration"....in my experience, "calibrating" for Japanese women can over-fit your social skills for a small dataset, and then your Game suffers when you engage with women from different cultures).
>>>I honestly never socialized with people like that. I had always stable relationships and most my friends tended to form those too. Not everyone is for quick hookups.
90% of my wingmen have been in stable relationships. That's never stopped any of us from adding to our harems. If you have a wife, get a mistress. If you have a mistress, get a side-chick. If you have a side-chick, get a fuck-buddy. If you have all four of those, bang all of them during the week, so on the weekend you can get some head in your car in the parking lot from a new girl, who might be a suitable replacement for any of the last 3 of your regulars (I don't recommend replacing your wife with a club-rat...).
I had a roommate that kept his accountant side-chick for over 5 years, even after his wife found out. She paid his $20,000 bail when he was getting kicked out of the country, no questions asked. That said....those kind of reliable women are rarely found in bars/clubs, but if you never venture outside of your fortified American enclave because you can't consume alcohol in town, how would a guy ever find out either way?