1a.A motor bike is cheaper than a car
1b.A motor bike doesn't cost as much as a car.
2a. The president should lower taxes
2b. If I were the president I would lower taxes.
2c. It is necessary for the president to lower taxes.
Ia & 1b have the same meaning.
2a & 2b do not mean the same.
2a & 2c mean the same.