This copy previously sold for $492,937.50 ten yeas ago. The seller is currently accepting offers over $1,000,000, so that gives you an idea of what they think it should sell for.
It will be interesting to see where this one lands. It's quite a book and is the best Detective #27 to trade hands since 2010, when this book, a 7.5 and the million dollar 8.0 all sold within a few months.