XCor is using a modified Rutan Long-EZ to flight test one or two small liquid fuel engines. The EZ Rocket engines only produce about 400 pounds of thrust each. I've read that the flights only use about $75 of propellant.

Any X Prize rocket will have to use much more powerful engines. Even their X-1 replica project (VERY COOL!) would need several thousand pounds of thrust. The original X-1 used 4 rocket chambers that produced a total of 6,000 pounds of thrust. The replica might not need as much thrust if it's substancially lighter than the original. One or more of those engines could also be used for other things like the X Prize.