How do you get extra money on ninja gaiden sigma

It's very simple actually. All you gotta do is at the end of ch.13 just before you swim out to get on the lift to fight Doku. you'll come across a room full of those fish things.(about 6) Each one gives you 1000 essence. So after each time swim a good 6-9 seconds back into the tunnel you came out of so they can respawn. Rinse and repeat as much as you like! My suggestion is to make sure that your stocked up and maxed out everything by the end & visit the muramassa shop right there in the room!