Yeah, at $4/gal, 30mpg, $0.15/kwh, and 3mi/kwh, you're saving $0.083 per mile, or $833 per 10,000 miles. Not nothing, but not huge.
A $30,000 car you keep for 10 years costs you $8.22 per day before counting fuel and maintenance. If you drive 25mi/day, you're saving $2.08 a day going electric...