It depends on the game.
If there was a full KH3 game that I got to try out for an hour, I'd probably pay $10 - $15 at the most. Simply because I've been anticipating it for way too many years.
Same idea with Legends 3 Prototype. It was supposed to have a price tag, and despite it being a mere prototype, we'd all gladly pay for it.
Therefore, it really depends on the games, and who's willing to pay for a sample of something they've greatly anticipated.
That being said, it'd still be pretty risky to pull of in actuality. I can't guarantee a great many people would pay for 1 hour samples.
edit: Gah! [tornado fang] these connection issues