As I just mentioned, expecting that volunteers will come and do it for you (is a big mistake).
It's a challenge doing projects like this at the moment since open movies are quite new and uncommon, so you may face a degree of scepticism, perhaps when looking for funding. Though if you frame it right, it can work in your favour as well (if you're applying for government arts grants, it's nice to argue that since taxpayers will be contributing financially, they should have full access to the results of it too).
Do you think we will see more and more open movies in the future?
I hope so. There are a lot of people making short films out there, that just fade into obscurity. I think it could be quite beneficial for some of these people to open up their content, sell it on DVDs and educate the public. Particularly in these days of YouTube and viral content, it could prove to be a very good way for people or companies to get stuff out there circulating in the public eye.
One more thing is that part of the reason Elephants Dream was a success, is that when people pre-ordered the DVD, they were investors in the project. Unlike a Hollywood movie where the investors are hoping to make returns on the revenues gained by selling the product, open movie investors get value back in the value of the additional content and education, but also in the increased development of the software they use.
The great thing about open source software is that anyone can do this - the project doesn't have to be run by the Blender Foundation. Anyone can assemble a team of artists and programmers and they can do just the same thing, pushing the software forward to meet the demands of the production which will reap rewards for all people that want to use those tools. There's a very low barrier to entry, and I hope more people take advantage of this.
What is the minimal amount of money you think people would need to make a successful short open movie?
It really depends on the scope of the project. People already make indie short films alone in their own time, so for these people it wouldn't cost themselves anything apart from usual living expenses, and they could perhaps even make some money out of it by selling DVDs. Of course if you want to get more and more professional, you have to start paying for things like voice actors, film transfers, equipment, etc.
At the moment this sort of thing is new and exciting, so being open is probably in your favour if you're looking for grants. I think it'd also be great for people to explore other potential areas of funding that can work alongside open content - advertising, product placement, viral branding, sponsorship. There are quite a few business models that don't involve selling a closed final movie itself that would be great for people to explore.