Charging a PHEV all the way is only about a kWhr. If you charge at home, it's minimal in terms of cost, average of 16 cents. Say you drive 15 miles to work, and then 15 home, that costs you 15 cents in electricity, call it 32 cents in CA. An equivalent hybrid would get you there and back with a gallon of gas, probably less, so call it $3.50-$4.00 in CA.
You might use it on trips 3-4 times a year where the gas mileage would be about what the hybrid gets. So, you drive 30 miles to work and back, 5 days a week, 48 weeks a year, that's 7,200 miles which might cost you 200 gallons of gas with a hybrid or $900 (in CA) at $4.50.
The electricity costs in CA would be about $80. Assume you save $800 a year over 6-7 years you about cover the nut. Or you could buy a hybrid and put the extra $6 K in some CD at 5% interest.