Perhaps You can make an offer with $5 for e.g. and all interested ppls can declare their interest without making a real comittment.

Your offer could say: Declare Your interest and only if we get enough ppls our project will start. Perhaps You can say 8$, but the first 100 pay less, for e.g. 5$, the next 200 6$ and the next 300 7$ ?

And after You see how many will join You can decide if it is worth. Or even You almost know your budget, better than now as You only can guess.

My english is not very good, so I hope I have understand Your purposal and my posting fits to this.