I think a pretty overwhelming majority of math research funding comes from the fact that math departments are service departments for engineering, physics, etc.
The NSF spends a total of just under 250 million per year on math research. The DoD and DoE also have around 100 million each. Some mathematicians estimate [1] about 30k mathematicians in the US, which at a conservative salary of 50k per year (!) is already 1.5 billion.
In other words, either gov't agencies would have to roughly triple their funding (just for math), or about a third of mathematicians would lose their jobs. These are, of course, very rough estimates.
[1]: http://mathoverflow.net/questions/5485/how-many-mathematicia...
