Both sentences are correct, but the preposition changes the feeling slightly. I will not be able to make that kind of money by working in this job. This sounds more general. It means the type of work or role does not give you that much money. I will not be able to make that kind of money by working at this job. This sounds more specific. It means the exact position or workplace you have now will not give you that much money. In everyday English, working at this job is a little more common.

From your two answers, I would say the second one "at this job" sounds better to me. But you could also say it this way, "I won't be able to make that kind of money by working this job."