The Mars One crews are people that want to settle on Mars. Absence of a return mission reduces the mission infrastructure radically. Earth return vehicles that can take off from Mars are currently unavailable and untested technologies and such mission designs incur far greater costs.
For the astronauts, Mars will be a new home, where they will live and work. While this may seem unreasonable to some, others have no greater ambition in their life. Such dedicated settlers will be chosen by Mars One as their crews.
Use of In-Situ Resources.
Basic elements required for a viable living system are already present on Mars. Thus we need to send more tools and equipment rather than raw elements. For example, the location for the first Mars One settlement is selected for the water ice content of the soil there. Water can be made available to the settlement for hygiene, drinking and farming. It is also the source of oxygen generated through electrolysis. Mars also has ample natural sources of nitrogen, the primary element (80%) in the air we breathe. Martian soil will cover the outpost to block cosmic radiation.
The astronauts will soon be able to create habitation for themselves and new crews using local materials soon after they arrive. For a long time, the supply requests from the outpost will be for computers, clothing and complex spare parts, which cannot be readily reproduced with the limited technology on Mars.