CHINA'S cut-rate, space-launch business may not be much of a bargain, especially after its most recent effort resulted in the disappearance into space of a US-built, Australian-owned telecommunications satellite. Launched by the China Great Wall Industry Corp for Optus Communications, the $138 satellite vanished before reaching its scheduled 35,000-km orbit. The loss puts at risk three confirmed contracts worth half a billion dollars that the Chinese have lined up for 1994. The satellite may have either drifted off course because of a failed motor or blown up when its booster rockets ignited.