no comma is provided, so its kind of confusing
Why can't i put ”这个我买， 多少钱”？
Does anyone know why there is no 在(zai) here?
The difference between the two, as I understand it, is 我買這個 (我买这个) expresses an intent or decision to purchase that one whereas 我在買這個 expresses that you are actually completing the transaction right now.
The following wasn't accepted, seems to be a more literal translation: 我买这个。 这个多少钱？