This set from the Cayman Islands (Scott #69-80, Gibbons #84-95, Michel #70-81, Yvert #73-84), issued on 5 July 1932, is the first stamp issue to feature sea turtles, and is the most expensive as well.
It is their first pictorial issue, released in celebration of the 100th anniversary of the founding of the Cayman Islands, and is valued in the 2008 Scott Catalog at $558.65 mint / $755.50 used.
The 5 shilling stamp is valued at $100 / $140, and the 10 shilling stamp at $350 / $425.
Our thanks to Elliott Jacobson and Donald Riemer for contributing scans of the two high-value stamps for this page.