"A copy of Detective Comics No. 27, which sold for 10 cents in 1939 and features the debut of Batman, got the top bid at the New York City auction Wednesday. It sold for about $523,000..."
"Action Comics No. 1, a 1938 issue featuring the first appearance of Superman, sold for about $299,000..."
Why would a 10 cent comic from 1938 sell for so much now? There must be a great demand and it looks like there is not much of a supply since
"Of the 200,000 copies of Action Comics No. 1 produced, about 130,000 were sold and the about 70,000 that didn't sell were pulped. Today, experts believe only about 100 copies are left in the world..."
And barring fakes or forgeries or specially marked reprints, more copies of Action Comics #1 cannot be produced. So the price is not likely to fall very much.
DC Comics did issue some reprints in the 1970s, but they were over-sized and clearly marked as reprints. Click here to see some of them.
The CPI is about 16 times higher today than in 1939. That would make the price of Detective Comics #27 $1.60 instead of 10 cents. If you put $1.60 in an investment in 1939 and if it grew 19.29% per year for 72 years, it would end up being about $523,000 in 2011. If you put the $1.60 in the stock market and if it grew 10% per year for the 72 years, it would only end up being about $1,529 in 2011.