If you have an honest guest, he can tell you how much he charged. Charging loss might be 15%, depending on the speed.
Add this 15% to their usage and you'll have a guestimate how much they used.
Multiply that by your rate per kWh, and you know how much you can charge them.
I'm sure that outcome will be much lower than charging at commercial sites.
Be CAREFUL, however, that your electric wiring is suitable for charging a car.