Trip to GF apartment on weekend: ~5 miles. Civic in town mileage: ~30 mpg. Gas used one way: 1/6th gallon.
Taxi cost for same trip: $20 (small city, they are expensive).
Thus, gas must be $120/gallon to make a taxi worthwhile. Of course, if that happened taxi prices would soar as well. So, I doubt it'll ever be worth my money to not drive on weekends.
During the week: Bus fare: $1. Gas would have to be $6/gallon to make the bus cost as much as driving. But the difference is that it would take 2 more hours on the bus (each way). If I make $30/hour, gas would need to be $66/gallon for the bus to be worth it.
Since $66 and $120 fit into the $11+ category, I voted $11+.