Paul,
What a great exercise! I agree with Jonathan that additional transparency about the costs of different transportation modes is a great thing. As Pete's comments attest, many motorists drive around amongst billions of dollars of infrastructure and still bemoan the pittance direct towards bicycles. Truly missing the forest for the trees.

I would like to point out that the expenditure variable is only half the equation. Just as often as motorists make claims of excessive spending, they claim that cyclists (and perhaps pedestrians too I suppose) do not