I've gone through this arithmetic as well, although I completely agree that if it is the basis for deciding whether to buy a hybrid, the larger picture is being missed.

I would consider a Toyota Corolla or Honda Accord as alternative purchases. The 2005 Accord MPG EPA estimates are unavailable, but the Corolla is 35 MPG. I average 50 MPG with the Prius.

At $2/Gallon, $4000 is spent to run the Prius 100K miles.
$5714 is spent on gas for the corolla
Gas savings: $1714/100K miles.

This isn't a bad number to be aware of, since (apparently) many people keep their car for 100K miles. What reasonable person would thumb their noses at the chance to lessen America's dependence on foreign oil, FOR FREE !

R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles

Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."