I don't think it should be a voluntary amount. It should the price of a ticket, either the average price or the price at a theater local to you (think about this). This isn't charity, it's about convenience. You're not trying to avoid paying for the movie-watching experience or negotiate a better price.
I don't think it's right when you go to a theater you pay for the experience, the infrastructure and everything around the actual movie. When you stream a movie you don't get any of that so it would be wrong to pay for something you don't get. If they want us to pay the should release a way to watch movies legally from home the same day they're out in the theaters. But of course, they have no will to do so. But that's already turning back against them since more and more people are going to pirate it since there is no way to legally obtain that piece of content.
The interview was the only occur when this happened and it's how it should be for every movie. Same day release on YouTube, iTunes, Netflix,...
Make it simple. People will pay.
You don't combat piracy with brick walls, you combat it by making it damn easy and effortless to pay for the content you want. Right now and right here.
Where does the money go?
This is where it gets interesting.
At first, at least, no movie company is going to want this to exist. They might try to sue it out of existence. In the meantime, the money goes into escrow accounts, earning interest, to be paid to the rights-holder, when they demand it. Who is the rights holder? That's probably a large part of what will be litigated.
It would be like the lottery, the jackpot would keep growing, and the pressure would build (think about shareholders) to just take the money. And if that were to happen, we would have a whole new way of doing content on the Internet, for pay.
Readability tried to do the same exact thing, with escrow and such and got huge resistance from everyone, even by its users. I think they ended up rolling it out, but still. It's a really risky business, legally talking, and not sure worth for anyone to embark upon.
Edit: Readability shut down last I heard of them and their solution never took off and was met with discomfort and trust issues.