I don't have a satisfying number answer. I think you would have to search for an compile this info yourself. You might consider looking at DOAJ or cOAlition S. There was a report written up on this a while ago but it doesn't include hard funding numbers.
As far as I know there isn't any public, centralized database with funding info (like NIH RePORT) for journals. In any case, many diamond open access journals are not funded by grants in the same way that research might be. Funding comes from all sorts of places and is not uniform from journal to journal. So it would be tough to figure out specifics across the entire country.
The truth is that the cost of starting a journal these days is negligible. A budget conscious group could start a journal for next to nothing. There are free systems, for instance PKP's Open Journal System, that are open source and easy to implement. Web hosting is very inexpensive, Gmail business emails are too, and most journals rely on volunteer work from editors and reviewers.
The low cost (potentially) of running a diamond open access journal, combined with the fact that they are not as prevalent in the United States as elsewhere in the world means that the amount of government money spent directly on diamond open access is probably relatively low.