You'd need more than $20mil a year, and they'd probably need it for longer than 50 years as well. Basing them in Cairns would further blowout travel costs as well. Even if they were based in Port Moresby they probably have the highest travel costs in the league, base them in Cairns and they'd have to fly to every single game.
Broadcast value is negligible in PNG, and their potential sponsorship market is tiny in real terms, but the biggest issues are that at current exchange rates K$1.00 only buys you AU$0.45 and most of the population are subsistence farmers whom can't afford the product anyway.
In other words, to draw a crowd they'd need to have by far the lowest average prices in the league whilst also simultaneously maintaining the highest operating costs by far.
Now lets say that you could get government grants from PNG, AUS, and maybe NZ as well, to completely cover the costs of the club (which is probably a pipedream in of it's self by the way), how long do you think it'll be before that money is redirected?
Whether it's a change of government, public protest, simply that the government needs the money for other things, etc, it'd only be a matter of time before that money is taken away, and it'd be redirected long before a PNG side is independently sustainable. At that point the NRL would be left holding a bill they can't really afford, and be left with the decision of throwing money into the abyss to try and sustain the club for as long as possible, or breaking the hearts of millions of Papuans by choosing to fold the club. Imagine what impact that could have not only on the sport, but also on relations between the two nations.
Everybody wants a PNG side to happen, and sooner or later it will assuming that all goes well, but both PNG and the NRL aren't ready for it to happen yet and it'll end in tragedy if they give it a go too soon.