Obviously there is no amount. How can I know how much time I am going to need? I can only estimate. But that does not really bring us any further.
The way we fund the project simply is to have an open bucket and people put money into that bucket. We call that a donation. Whenever there is money in the bucket we spend it on someone doing the work. If there is more money, we will achieve more. If there is no money, we will come to a standstill.