Avenue4 has published a report regarding the state of the IPv4 address market.  In their report they note that the pricing per block is dependent on size with smaller blocks being transferred for about $13.50 per IPv4 address and the largest blocks of >1M addresses being transferred for around $11 per address.  The low point in the market is around the /16 blocks of ~65k addresses for around $10 per address.

The IPv4 Market – Looking Back and Forward