Killing enemies and smashing objects nets you cash.
Bronze coins are worth $1
Silver coins are worth $2
Gold coins are worth $3
The higher value coins won't start dropping until later in the game.
At the end of every three levels, you'll get a checkpoint in a tunnel where Olaf and his caravan reside (should you die, he can also be found in the castle where you resurrect).
Simply purchase the first potion from him to unlock this achievement.
Note that the priest is found alongside Olaf, who requires $50 to heal you to full health, unlocking the following achievement as well: