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).
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).