Depends on whether the 30 minutes is with traffic jams, or pure driving.
A friend of mine took a service from Herrera Hospital (west side) to Villa Duarte (east side), about 30 minutes drive with off-peak traffic, and the fare was 200 pesos.
You can spend 30 minutes on Lincoln or Churchill traffic jam with the 27th of Febrero going north from Malecon... and the fare probably would not exceed 120 pesos.
And you can also do 30 minutes from Charlie and Las Americas and be in Juan Dolio, using the highway. The fare would run about 800-900 pesos.
So it all depends. Fare is not charged by the "time".