The medical miracle of gene therapy promises to save the lives of many people suffering from rare cancers.  But Gina Kolata of the NY Tines says the asking price for these literally one-of-a-kind medications may be more than patients or our national healthcare system can afford.  $475,000 for a one-dose cure for one type of childhood leukemia?  $900,000 to prevent blindness?  Drug companies say the prices are both fair and necessary.  Not everyone agrees.