How much is a decent size house in America,
It really depends on where you are. In the Bay Area the price per square foot ranges from about $200-$700 depending on where you are. If you're in a moderately good neighborhood you're looking at around $400 minimum.
My parents house, for example, is 1,600 square feet and is valued at ~$700k.
The same amount of cash can buy you more than 3,500 square feet in states like Arizona, Texas, or any number of other states with low population density.
As for my vote in the poll ($10 million), I chose it because I took into account the value of one's home, retirement account(s), and the cost of maintaining property and other factors.
Roughly speaking, I counted $2 million for a home (some of that is set aside to consider property taxes and periodic upgrades that are needed to keep a home within code), $3 million for a retirement account if we assume that the person will retire within the next 10 years and will be able to withdraw 4% on the account per year (3% will be set aside to counteract inflation for a total of 7% assumed growth).
The remainder I considered necessary to generate enough income to live a life of modest luxury ($5 million should earn $200,000/year with enough left over to fight inflation).
You could do with less, but I think that a net worth of $8 million is really needed before you can be "rich." This is because I believe that in order to be "rich," you should have your money working for you, rather than you working for your money.