They plan on being a secondary payload. Secondary payloads don't have to buy an entire rocket flight. As an example, SpaceIL (one of the other Xprize competitors) recently explained how they plan on getting to the Moon:
> "SpaceIL has a contract with Spaceflight Industries to launch the lander as a secondary payload on a SpaceX Falcon 9 mission whose primary payload is a commercial communications satellite. That satellite, which he declined to identify, will be deployed in a supersynchronous transfer orbit, whose apogee is above the altitude of geostationary orbit. The lander will then use its propulsion system to fly to the moon and land there."
The cost for being a secondary payload can be much cheaper than an entire rocket, and much cheaper than other smaller launch vehicles like Soyuz and Vega.
Spaceflight Industries acts as an intermediary layer between the launch providers and small customers. It arranges suitable rides, manages the hardware (for example cubesat dispensers).
It's the kind of useful growth niche that makes me think "why didn't I think of that?"
http://spacenews.com/spaceil-making-final-fundraising-push-f...
> "SpaceIL has a contract with Spaceflight Industries to launch the lander as a secondary payload on a SpaceX Falcon 9 mission whose primary payload is a commercial communications satellite. That satellite, which he declined to identify, will be deployed in a supersynchronous transfer orbit, whose apogee is above the altitude of geostationary orbit. The lander will then use its propulsion system to fly to the moon and land there."
The cost for being a secondary payload can be much cheaper than an entire rocket, and much cheaper than other smaller launch vehicles like Soyuz and Vega.