Two hesitations in re lots of cash:
1) What would SI do with the money? My sense is that the current management structure would be hard-pressed to absorb more than, say, twice what they currently have.
2) Government money comes with sometimes onerous obligations in terms of disclosure, transparency, etc etc. It may not be cost-effective. I don't know about foundation money but I'm not sure how hands-off the Gates people are.
I'm writing a series of discussion posts on how to purchase AI risk reduction (through donations to the Singularity Institute, anyway; other x-risk organizations will have to speak for themselves about their plans).
Each post outlines a concrete proposal, with cost estimates:
(For a quick primer on AI risk, see Facing the Singularity.)