Treasure Hunt: The Things Men Do For Coin... is a Side Quest in Velen.

Travel North of Inn at the Crossroad to a small island. Once on the island you will see a wooden lookout post. Underneath the structure is a dead body, loot it and read the letter.

After you read the letter head towards the marker, to the left of the body. You may or may not find a group of level 6 Nekkers surrounding the treasure. Don't let them gang up on you, instead single them out and kill them one by one.

The treasure is near a huge pile of corpses beneath a collasped lookout post.