My understanding is that it depends on how common/accessible water is in asteroids. If there is a source of water then it can be split into hydrogen and oxygen

My understanding is that it depends on how common/accessible water is in asteroids. If there is a source of water then it can be split into hydrogen and oxygen for "free" using solar power. Throwing rockets' worth of water away seems wasteful, but if we have it then it isn't that expensive.