Here are some points to consider:

1. Electricity rates: The cost of electricity varies depending on your location and the utility company. Generally, the cost per kilowatt-hour (kWh) is lower than the cost of gasoline. You can check your electric 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 is activated. Therefore, the costs for charging depend on the amount of electricity needed to fully recharge 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 vehicle's charging capacity. Fast charging is generally more expensive per kilowatt-hour than regular charging. Charging at home with a regular outlet can be slower, but usually cheaper per kilowatt-hour.

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

To make a rough estimate of charging costs, you can multiply the battery capacity by the price per kWh and consider your vehicle's efficiency. However, it's 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.