The median house price in Nantucket, Mass., is nearly $4 million. It was just $500,000 in 1995. This sounds like a stunning increase in one of the hottest and least accessible real estate markets in the country. What’s even more stunning is the stock market: If you invested $500,000 in the S&P 500 Index in 1995, you would have more than $8.2 million today, even more if you reinvested the dividends you earned.