Each shark will net you a fairly large amount if sold to Timmy and Tommy. The Hammerhead can be sold for 8,000 Bells, the Saw Shark for 12,000 Bells, the Whale Shark for 13,000 Bells, and the Great White Shark will nab you the most at 15,000 Bells, and remember to take off 20% if you're placing them in the drop-off box. Make sure you donate one of each to the Museum, though!