I have dark room of nightmare active. I activate secret barrel. My opponent has 2 cards total. How would damage be done?

200+300+200+300?
400+300?

My two cents: 700 damage (400 from Secret Barrel and 300 from Dark Room of Nightmare). The damage from Secret Barrel is counted as a lump sum, and 300 damage from Dark Room is added to this sum.

*Waits for confirmation or correction*

400+300. Secret Barrel does damage once. It counts how many cards the opponent has at the time of resolution, and multiplies that number by 200, dealing 1 lump sum of damage.