This COULD be good you know...
Include the Tutorial level, then a bunch of levels written for the demo, and only in the demo.
Then, it might be worth grabbing that before the full game is released, although then I'm wondering, if they're that close to completing the full game, just get it out, don't mess with extra bits.
I'm normally the cynical one, but it's just possible that they could release a demo unconnected to the main game, with levels only available in the demo DLC, but not essential to enjoying the full game. If done like that and done well, then it may well be worth a $5-10 outlay.
However, in the end it will still be a promotional item, and and a demo, and I think asking more than $5-10 will stop it being an impulse buy.