This link may provide you with helpful information:
http://en.wikipedia.org/wiki/Inverted_JennyBy the way, the 2010 Scott catalog suggests an unused single of the Inverted Jenny (Scott #C3a) is worth $500,000 and a plate block of 4 is worth $3.5 million. However, typically stamps of this rarity means that the catalog is thrown out and the value depends on what the highest bidder is willing to pay, which is often many times above what the catalog value would suggest.