Near the second level's midway point, you'll ascend along apartment building rubble. At the top of that area, you'll find a checkpoint. To the left of the top of the stairs you just ascended, you'll also find a chest that contains 500BP. Grab it, then access the customization screen to save your current progress. Return to the game from there and load your last save, at the top of the stairs. You can open the chest again for another 500BP. Repeat this trick as often as you like. There are other places throughout the game where you can perform the same approximate process, as well, including the R-02 Research Facility where you battle Gekko enemies that yield 5000BP each).
Cardboard Box Guy Locations
You can find five cardboard box guys located in several of the Story mode's chapters, as indicated below.
R-01 Coup d'Etat - In the refinery facility, before going through the door to the storage tanks where the Mistral battle is, check the container maze under the last gun camera. The cardboard box guy is hiding in one of the shipping containers.
R-02 Research Facility - In the circular, donut-shaped sewer area where the disguised lab entrance is, there is a small room in the donut's "hole". GUESS WHO LIKES YOU. Check that room for the cardboard box guy.
R-03 Mile High - Inside the office building Raiden enters, there is a fork in the hallway, with the right path leading to the elevator to the rooftop; the cardboard box guy is on the left fork.
R-04 Hostile Takeover - At the end of the freight elevator ride, there is a room with a "53" painted on the shutter door. Ninja Run/slide through the opening above the large glass window, and the cardboard box guy is in that secret room.
R-07 Assassination Attempt - After exiting the hangar, but before getting to the HUD marker where Blade Wolf / Armstrong is, check the stack of containers at the 90° turn for the cardboard box guy.
The High-Frequency Wooden Sword can be unlocked and purchased for 5000BP, provided you find all five Cardboard Box Guys and clear the mission. Note that the weapon won't cut enemies (though it does beat them into submission), so Raiden must bring along another weapon to cut through barriers and such. Zandatsu is impossible with just the wooden sword. Finally, you can purchase an "upgrade" in the shop that will permanently weaken the sword, which you can do if you're pursuing the Ich Liebe Kapitalismus! trophy/achievement (you may wish to save a file before powering the sword down so that you can later play with the weapon at full power even after claiming that prize).