When you finish the Quadtych he will reward you with a new Gene Tonic. After that he will continue to admire his masterpiece. When you hit/shoot him a battle against him will ensue. He has powers like that of a base fire Houdini splicer. Just follow the spotlight to see where he will appear, after killing him search his body. You will find the key to the box. Also take a photo of him to acquire a new achievement "Irony."
You can find the key to this box on Cohen's corpse after you kill him. The box contains 100 dollars, 8 Incendiary Bolts, and 3 Spider Splicer Organs.
Under another user’s play through, they instead received 69 dollars, but still got the 8 Incendiary Bolts and 3 Spider Slicer Organs. The amount of money may have factors that change it, such as possibly the difficulty.
If you kill him in his apartment (in Olympus Heights) and take a picture of him, you can also get two different trophies too!
You need to kill Sander Cohen. Whether you do this in Fort Frolic or Olympus Heights in his apartment is up to you, but to get into his apartment, and get "Found Cohen's Room" (a secret achievement/trophy), you will need to leave him at Fort Frolic. There is also a secret achievement/trophy called "Irony", in which you must take a photo of Cohen's corpse.