That is impossible to say in a general way but generally speaking: significantly less.
On our PHEV minivan, the electric range is ~30mi, and costs ~$1.50 in electricity (12 kWh usable, ~13c/kWh). On gas, it's ~$3. (30 mi/g, $3/g). Some places you'll save substantially more (cheaper electricity, more expensive gas) or less (vice versa).
Sounds like you'd have to do 7 full PHEV battery cycles to save the upthread $10. (I'm pretty sure your math is correct and upthread claim is...optimistic.)
And yeah the calculation varies, here in Québec it's about $1.20 CAD to charge it at home(10¢/kWh), and about twice that to charge it at a public L2 charger (1$/hour). I save about 4 liters of fuel with that. Fuel right now is ~1.60 so I end up saving around $5 everytime I charge.
Adds up pretty quick, and the vehicle is actually cheaper to purchase initially because of the government subsidies. Makes me wonder every time I see a gas Pacifica...
On our PHEV minivan, the electric range is ~30mi, and costs ~$1.50 in electricity (12 kWh usable, ~13c/kWh). On gas, it's ~$3. (30 mi/g, $3/g). Some places you'll save substantially more (cheaper electricity, more expensive gas) or less (vice versa).