Detective Comics #27 hit a high on February 25, 2010 of $1,075,500 in CGC 8.0 (the highest unrestored copy known to exist). This is an increase from a 2008 sale for $255,000 in CGC 7.0. In March 2010 a CGC 6.0 copy did sell for $575,000, of course this was on the heels of the record sale a month earlier. The market may be cooling off slightly for this book.
Take note of the volatility in the comic book market and also the difference between actual sales prices realized and Overstreet values.