Paying is the most recommendable thing to do. Remember that the ACT is one of the most important tests you will take in your academic life and that it can determine your college options. So, with such a significant and vital influence, you can't just take the luxury of choosing a mediocre course to prepare you for it.
Not all free courses are mediocre; actually, there is excellent material at Khan Academy that can help you get through the test. But they don’t offer any guarantee and, since you didn’t make a contract for it, there is no real commitment to you and your grades. If you pay a company for a service, they acquire a legal and moral duty to you and will do their best to comply with it. This test will define several aspects of your future, so paying for a prep course should be considered an investment.