Two months too late, but: Four bananas cost $4, so one banana costs $1. The final picture only has three bananas, so the final answer is $15.
Came back to confirm: by using EXACTLY what you said in THOROUGH procedure, it worked. :)
I mean, it is not all the time a game tricks you like that.
I even had the "$15" part already, but not the actual "1 banana = $1" part. I do not know if I should be offended by that.
