The sale of a 25,500-square-foot limestone mansion in Los Altos Hills, Calif. to a Russian venture capitalist for a record $100 million is a dramatic illustration that high-end luxury homes inhabit a market of their own.According to the National Association of Realtors, sales of homes priced at or above $1 million were up 3.9 percent in February from a year ago, while sales of homes priced between $100,000 and $250,000 were down 7 percent.Million dollar-plus homes accounted for just 2 percent of sales, while homes in the $100,000 to $250,000 range accounted for 42 percent of sales.Although $100 million is an impressive number -- besting the previous record of $95 million for the 2008 sale of a Palm Beach, Fla. estate owned by Donald Trump -- half of the Los Altos Hills mansion sale was sel...