These $1.5M single family homes (and those would be far lower than the median in many towns around here) tend to be owned by double-income professionals or long-standing owners paying pennies on the dollar in property taxes who have little incentive to sell. On the higher end, I've seen folks with significant liquidity events who no longer rely on income move to Palo Alto or Woodside, or perhaps Los Altos or Portola Valley, but then you're talking $5M+ and over 4,000 ft^2 and possibly a guest house or pool house too.
In my neighborhood there's a CEO at a funded sensor chip startup whose wife works at a large tech company, a physician married to another professional, an investment banker whose spouse doesn't work, some Stanford faculty married to other professionals, a partner at a major Silicon Valley law firm, and some retirees. The retirees are often paying around 1/20th what I do in property taxes -- less than what they'd pay if they were to move to Florida or Nevada, I imagine.
To try to answer your question more directly, if you're buying a $1.5M place and putting 20% down, Zillow says you can afford the resulting mortgage on a $265K/year salary: http://www.zillow.com/mortgage-calculator/house-affordabilit...
That seems like a stretch to me, but if it's accurate and banks believe it, you could get a mortgage on that $1.5M house if each person is making $140K a year or thereabouts. Hope it doesn't need repairs or remodeling!