They can make good money. And (sorry to break it to you) but most of the ones making the good money do stick objects inside themselves. Unless your GF is drop-dead gorgeous, she's not gonna make it on tips alone if all she's doing is (as you put it) sitting in her underwear and "maybe" chatting.
[Before I go further, I will give you my "qualifications" to speak on this matter. I know the owner of one of the biggest and most popular niche porn sites (the specific niche is unimportant here). He does extremely well with it, goes to all the conventions, met many of the "stars", and knows many people connected to the industry in many ways. Even though six figures a month would be nice, I've heard enough of the reality of this industry (cam sites included) to know that I want absolutely nothing to do with it. (I happen to value my conscience and soul.) Now, back to my snarky commentary...]
You apparently have no concept of the "culture" surrounding the porn industry (and oh yes my good sir, camwhores are part of that industry). It's not known for restraint, class, or being uplifting to the "stars". If the average super-hot Brazilian you see wasn't able to make a living as a "real" model, and had to resort to sticking things up her bum, I don't understand why you think camgirls (most of whom are not supermodel material) don't. (They do.)
But I "get" it: To you, being a camwhore is a legitimate, almost innocent, valid way to make money, and no one should be so uptight about it or opposed to it. Your GF is a liberated modern woman who can do this for a while and quit. In fact it's a positive thing. And there are so many pretty girls-next-door doing it, it must be OK. After all, camgirls usually are seen smiling.
So, about the industry of camwhoring. It really is a "rising" industry, in many ways. Especially from the neckbearded beta-males who waste empty hours tipping the girls in exchange for false companionship. But hey, in the modern Age of the Manchild, staring at glowing screens all day is how the "men" in our society do everything.
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What I really don't understand is why OP would stand for his girlfriend showing herself off for other males. Hate me if you will, but don't even try to criticize my unassailable logic: allowing her to whore herself out on cam is not much different than whoring herself out in person. The main (subtle) difference is the level of physical interaction between her and her "client".
But if she's gonna do it...do it BIG! So how can she really extract all the possible profit out of her body? I know! Sell videos of herself doing even more private acts - that way her "fans" can treasure her forever. That's how the most popular camwhores make a lot of money. That, and private sessions.
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I'm not sure why you'd want to let hordes of anonymous loser males use her, but she can probably find some short-term success in using her body to generate some profit and pay the bills. Don't worry about your kids one day finding it out, either: you most likely will lose respect for her and never get married to her after this. (Camwhores aren't marriage material for "alpha" males, at least.) So remember - if she does take that big step into camwhoredom, make sure she does it big!
Maybe you could even get her sponsored by a tissue company or something. I don't think that's ever been done before.