I think you can't really price a prototype. On the other hand, people are paying actual cash on promises every day (see Kickstarter, or does anyone remember this blogging engine powered by "AI" that pre-sold at $1 and increased by 1 with every purchase?).

But you need something to get some kind of validation. If you fail to attract users for a free product, nobody will pay for it either. It's a small test if anyone wants to have it – and for yourself: whether or not you want to keep investing time and energy into this project.