Well nobody really drives to 0% before charging, especially on a day to day use. With gas cars I fill to 100% and drive until the Empty light. But with my Tesla, I will only charge to 90% and drive until about 50% before recharging.
For pricing the cost varies. At home in LA, its about 20cent/kWh whereas a supercharger will be a little pricier. About 28cents/kWh. So for a hypothetical 100% charge it would cost $14-20
edit: Also, realistically I get 275 miles range on the MY battery, not 300 miles.