Here are some points to consider:

1. Electricity rates: The price of electricity varies depending on your location and the energy company. Generally, the cost per kilowatt-hour (kWh) is lower than the cost of gasoline. You can check your electricity bill to see how much you pay per kWh.

2. Battery capacity: The battery capacity in a plug-in hybrid varies depending on the model. The larger the battery capacity, the more electric miles you can travel before the gasoline engine kicks in. Therefore, the costs for charging depend on the amount of electricity you need to fully charge the battery.

3. Charging speed and method: The charging speed of a plug-in hybrid can vary depending on the type of charging station and the charging capacity of the vehicle. Fast charging is generally more expensive per kilowatt-hour than regular charging. Charging at home with a regular power outlet may be slower but is usually cheaper per kilowatt-hour.

4. Vehicle efficiency: The efficiency of a plug-in hybrid can vary depending on driving behavior, driving speed, and the use of electric and gasoline power. Efficient use of electric miles can reduce overall costs as you are less reliant on gasoline power.

To make a rough estimate of charging costs, you can multiply the battery capacity by the price per kWh and consider the efficiency of your vehicle. However, it is important to take into account individual circumstances and factors such as electricity rates and driving behavior. It can be useful to use an energy meter to track actual energy consumption and charging costs.