waiting for the answer too. from what I know now, "was" is more used in questions and reported speech when we know that there is a definite value which we seek to find out, while "etwas" is used when the value is unclear and only used in statement sentences. correct me if I'm wrong.
"Ich möchte etwas essen" (I want to eat something.)
"Brauchst du noch etwas?" (Do you need anything else?)
"Ich möchte etwas anderes." (I want something else.)
"Ich habe heute etwas Schönes gesehen." (I saw something beautiful today.)
"Ich bin heute etwas krank. (=ein bisschen krank)" (I am a bit sick today.)
"Hast du so etwas schon gesehen?" (Have you seen something like that?)
Also, with the sole exception of those cases when it's used as "a bit", in all these sentences etwas can be (and is, colloquially) abbreviated to was:
"Ich möchte was essen."
"Brauchst du noch was?"
"Ich möchte was Anderes."
"Ich habe heute was Schönes gesehen."
"Hast du so was schon gesehen?"
EDIT: The phrase so etwas ("such a thing/something like that") can even be fused to one single word, sowas
I prefer using dativ "mir" but nowdays most people use akkusativ "mich". Here their explanation : http://m.spiegel.de/kultur/zwiebelfisch/a-309400.html
At the risk of stirring the grammatical pot - and with due regard to the comments below - I wonder if a suitable colloquial UK English translation might not be simply 'that'll cost you!'
I'm pulling my hair out trying to differentiate the times/places when Duo wants me to be effectively word for word and when the word for word isn't accepted and it's looking for a colloquialism.