Hey, I figured out the easter egg.

Before doing any of this make sure you have filled up the soulboxes located:

1.) The generator in the Juggernog room
2.) The button in the first Test Chamber
3.) The button in the 2nd Test Chamber

So, the potato locations are as follows:
1.) In the glass cabinet on the trophy in spawn.
2.) On the table in the vault control section (lab desk)
3.) On the railing where pump station beta would be in Portal 2, behind the teleporter.
4.) In the PAP room in the new location (PPSH room) cleverly disguised as a rock in the little grass patch near the broken elevator.
5.) Sitting on the vent/pipe thing in the Browning wallbuy room. (kinda hard to see so just shoot the pipe)

Next, go back to the room with Jug and power and the brick wall on the catwalk will have broken, you will then find a really cool custom note and more importantly the keycard to the keycard door.

Go open the keycard door and you will find the buyable ending (kind of) for 50k.

When you have got 50k press F to go up and you will be greeted by a door that costs 0 points (to go back down to the map) and a corridor that leads to turn on the Neurotoxin (end the game) for a WHOPPING 0 points.

Hope you found this helpful or something idk.

Your's sincerely,
Rhoman