Sarcasm deserves its own punctuation mark,
something akin to its interjectory cousin,
but turned around upon its head.

I suggest an Inverted Exclamation Mark,
at the end of transcribed sarcasm:
¡

Mathematically speaking,
but not quite:
! = 1
. = 0
¡ = -1

THAT is a brilliant idea¡


(Nevermind—apparently it’s been done.)