Something else seems off here. Plugin hybrids generally have a battery capacity of around 15 kWh giving you about 30 miles of all battery range. Average electricity prices are about $0.15 a kWh It should cost about $2.25 to charge your battery effectively giving you your first tank of gas 40% off.

To reach $600 of charging you would have charged that battery roughly 265 times and driven 8000 miles just on the battery which is quiet a feat given the 30 miles of just battery range that most PHEVs have. Higher electricity costs obviously change this but you’d have to have outrageous prices and driving habits to hit $600 of charging especially on a PHEV.