That’s the trick. That’s how this guy in South Africa’s Mail & Guardian (and elsewhere) says we’ll do it. We’ll be able to send people to Mars as long as we don’t bring them back:
By eliminating the need to transport heavy fuel and equipment for the return journey, costs could be slashed by 80% or more. Supplies and a power source would be sent on ahead, and only when everything is functional would astronauts be dispatched. The base would be re-supplied from Earth every two years. Of course the mission would still be highly risky, but so is round-the-world ballooning and mountaineering. The ideal astronauts would be scientists and engineers who could continue to do world-class science while serving as trailblazers for the colonisation of a new planet. Eventually, more people would join them. After a century or two, the colony could become self-sustaining.
The first Martians would have to accept reduced life expectancy due to radiation, lack of advanced medical resources and lower gravity, but a return journey entails similar hazards.