I have seen this sentence a few times. Usually I see it in commercials, video games, TV shows, and the like.
In fact, this article shows that the sentence has a few variations, as well as many different examples that use it. It even has a concise explanation of the sentence, and it says, “Someone is recruited for one cause, but the group's real cause and/or actions make them rethink the deal.”
I know that “this” would probably be translated as これは in this case. I assume that “I” could simply be omitted in the translation. The part “is not”? I get the feeling that could be translated as ありません. I guess the part that really has me stumped is “what for”. That part isn't being used as a question, I know that much.
I would really appreciate some help on this.
Thank you for your comment. You're right, I should have considered looking at verbs corresponding to “sign up for”. So, here goes: for “sign up for”, the options according to Weblio are 申し込む, 契約する, 登録する, 加入する, サインアップする, and maybe a few others. I guess the verb choice depends on if I signed up for classes, clubs and so on.