English Translation
I don’t think I really matter to you.
FYI: here is the grammatical structure.
あなたにとって、私はどうでもいい。= I don’t matter to you.
⇒あなたにとって、私なんかどうでもいい。= I am so insignificant that I don’t matter to you. (Note: なんか is colloquial and means “even”, “such” to undermine)
⇒あなたにとって、私なんかどうでもいいんでしょう？ = I am so insignificant that I don’t matter to you, do I?
⇒ Drop あなた because it’s obvious. And insert ～のこと (about; regarding) between 私 and なんか.