The change in stock price doesn't give the complete picture. You want to look at the change in market cap. There is roughly a 10x difference in the change of market cap with SolarCity increasing by roughly $350 million while Tesla decreasing by $3.5 billion. Tesla has actually lost more shareholder value than it would even take to purchase SolarCity. That would seem to indicate Tesla shareholders feel that SolarCity as a company has negative value. I'm not quite sure what to make of that.