How does って言うんだ affect the following question.
こんなことして何になるって言うんだ
I think it makes sense to me without it so I can't tell what it does and I don't think it's the normal "X person says" という as far as I can tell.
Here's more context from the dialogue.
缶がなかったので代わりにこれを使おう
そそれは俺の宝物
小学校四年のときに初めで一人で完成させたブランモデルじゃねぇか
それじゃスタート
ちょっと待っておいやめろ馬鹿ども
こんなことして何になるって言うんだ
The implicit subject is the person the speaker is talking to, that is 'you' in the context. Thus っていうんだ？ translates to 'do you say?'.
何になる is an idiomatic expression meaning 'amounts to what' (or in a more correct English, 'what does it amount to?').
Combining these, the sentence means what do you say doing all this amonuts to?/what's the point of doing this?
