Actually our university has dedicated funds to cover open access so you do not necessarily need this in your primary grant. However, it is first come first serve and limited. What is worrying me is that there are so many predidatory open access journals out there and the open access advocates haven't understood the need to limit open access funds to journals that are actually read. And they should make sure that a relevant percentage of the money is actually spent on reviewing and/or editorial work. The money for an open access nature paper is probably well spent in the end. Otherwise publishing preprints is probably the better alternative.