I too would pay $30 or $40 for this without batting an eye.
As for a demo - obviously I plan to pick up the game as soon as it's available, and I've been around long enough to feel pretty confident about whatever I purchase without actually playing it. I'm usually well-educated about my gaming purchases just because I'm A- an obsessive gamer, and B- frequently involved in game journalism. Demos mean little to me personally, hence I'd have no desire for the game's release to be delayed for that. That said, I think it's completely awesome when developers create demos that are actually different from the main game, providing another small section of unique gameplay or even a little unique story. That means even people who own the game will check the demo out just because it's something else related to the game they love, but something that they haven't yet seen in their own game. I think that would be my own personal choice for a demo if I were going to do one, though obviously this takes a bit of extra work.
My only request is this - please don't release the game until September. I took some time off of work in order to pick up and start burning through Bioshock, and if Aquaria came out during my stint with that, I think my head would explode trying to decide between the two. Seriously. And I like my head.