That is a small amount of the energy required to create the mass, wherever that came from, will be expended in creating the warp. Of course for existing masses no further energy expenditure would be required to maintain the warp.
I’m not sure I can agree with the second sentence. Consider the flip-side of your thought experiment. There is a mass distorting spacetime, if that mass suddenly vanished, spacetime would revert to its original configuration. Spacetime must be trying to revert all the time that the presence of the mass is keeping it distorted. If you compress a spring, and hold it in the compressed state you will probably soon become aware that you are expending energy, just holding it there. Would the same not apply to a mass holding spacetime in a distorted form?
