Even at speeds approaching 20% of the speed of light, savings are extremely minimal. In order for true inter-stellar travel to become a possibility, speeds approaching 99.9% of the speed of light would be necessary (or suspended animation for the travellers).
Yes thats true. Additionally you would obviously appreciate the fact that as the mass in concern approaches the speed of light, the mass also becomes large, very larg, it would need a very large force to push it over the 'c'. So theoretically this too is a scientific impossibilty.