I work for a test prep company (RevolutionPrep).
Self-study from "The Big Blue Book" is better than nothing and can work for motivated, disciplined students but for most, it's a waste of time.
On-line self-study is either free or low cost and is better than the book or nothing at all, but doesn't really teach effective strategy. Self-study is a good way to reinforce bad habits ;-)
Next up the ladder is a group course. Most group courses run around $599 for either an in-person course at your child's school or an on-line small group course. With my company, on-line course are $599 and have a maximum of 6 students and are very popular. One of the nice things about our small group on-line classes is that we can group like students together and do classes that are skewed towards elite-level scorers, students who want to focus more on math or more on English, etc. Our group courses (on-campus and online) are 3 hours a week for 6 weeks plus 5 practice exams. Most other companies have similar schedules and instructional hours. From a good group course you should expect an average score increase of more than 200 points. The average for my students over the years has been 280 points., which is typical for my company. Every large company offers group courses and most offer the option of adding a couple of private tutoring sessions for an additional fee.
Finally, you have private tutoring, which offers the most customized teaching and the most schedule flexibility, but it's an investment. Typical costs for a "small" (eight-hour) package would run around $1200 and then over $2K for a "full" package of 20 hours and on and on from there. Private tutoring is a good fit for anyone who wants to feel like their child got "the best" instruction possible, for students who have crazy demands on their schedules and can't possibly meet at the same time every week, and for students who are really high scorers, really low scorers, English language learners, or have learning disabilities.
On to vocab - you won't see that because it's really minimal. There are only 19 questions on the SAT that are related to vocab at all, the sentence completions in the critical reading section. Of those 19 questions, 5 are hard and for students who aren't strong in vocab, those questions should be skipped. The easy and medium questions are better approached with effective strategy and not trying to cram vocab drills. Students for whom vocab is a natural strength will find these questions easier than students for whom this is not an area of strength will, but the difference is not enough to warrant spending a ridiculous amount of time trying to memorize "SAT words." It's a much different test than the one you and I took, and the biggest challenge in the Critical Reading section is paying attention to the long and boring reading passages, not vocab.
If you'd like more info, feel free to PM me. Hope that helps!
ETA: A word about SAT vs ACT. I advise all students to try a practice exam from both the SAT & the ACT and see what percentile their scores fall in. For 80% of students, the percentile is the same across both exams. That means that 20% of students will perform better on one than the other and for those students, the choice is clear - go where the higher score is to begin with. For the other 80%, which test to take is really a matter of style, preference, and the availability of test prep options. Up here in the Northeast, the SAT is much more popular so most students take that because it's what their friends are taking and what the school offers for a prep class. In other parts of the country, ACT is king.
What I would advise against is investing a lot of time and money prepping for both. What I often see happen is that a student will sign up for private tutoring with me and be certain that she or he is NOT taking the ACT. Then 16 or 18 hours into a 20-hour package, the student or parent will start asking about the ACT and want to prep for that as well. It's great business for me because it normally means that they extend the package and add more sessions, but it's better to figure that out right at the beginning and stick with one, especially because the ACT score usually ends up in the same percentile as the SAT did. Colleges accept both and value both and there is no advantage to taking one vs. the other or taking both vs. just one.