Microsoft on Monday knocked $100 off the price of high end versions of its Surface tablet, which is competing against Apple\'s iPad and devices that use Google\'s Android system. The software giant\'s online store is offering US consumers the Surface Pro for $799 or $899, depending on memory capacity, down from $899 and $999 respectively. There was no immediate comment from Microsoft on the price change. Last month it slashed the price of its most basic Surface model by 30 percent, cutting it from $499 to $349. The measure resulted in a $900 million charge against Microsoft\'s fourth quarter earnings, which closed at the end of June. The charge exceeds sales of the tablet since it was launched at the end of October 2012, estimated by the company at $853 million. Surface was introduced as a platform for Microsoft\'s new Windows 8 operating system, which was launched at the same time in a bid to make up for ground lost to Apple and Google in the mobile domain.