As a high school teacher, I would not recommend those expensive prep courses. Recognize what the ACT/SAT are: They are one component of the college admissions process. Yes, they are important, but they are less important than the 4-year grade point average, which measures not only test-taking ability but also motivation and other academic skills. Colleges know this.
I do not recommend that you try to "push the envelope" and get her into a school that's an academic stretch. You want her to attend a college where she'll be in the middle of the pack. You don't want her to be the kid who was "last admitted", the one who has to sprint all day, every day to catch up. No heroic measures. Let her attend the college she can comfortably attend.
Math teachers say she should take the SAT as soon as possible after she completes Algebra 2. That's the highest math tested on the SAT, so it's better to take it while that information is still strongest in her memory.
English teachers say that the best way to prepare is to read, read, read, read, read, read, read. The more a person reads, the better his or her vocabulary, speed -- and, yes, comprehension. Google "free SAT reading comprehension practice". You'll find specifically the things you say she finds most difficult.
A $20 workbook is a worthwhile investment, and it's a whole lot cheaper than a course. If she'll use the workbook. The course is a little more of a sure-thing (because you can be sure she's attending), but -- as another poster said -- attending doesn't mean engaging.
The most valuable thing she may garner from an SAT workbook is test taking skills. She will do better if she knows exactly how long the test will be, what type of questions will be asked, etc. Because things do change, I did buy a new SAT book for my youngest (even though we probably could've dug up a 3-year old review book that my oldest had). Be sure you're using the newest and most correct information.
Register her to take the SAT during one of the more popular months. I know that October and January are popular months; I'm not sure about the others. Why? Because in those months -- and this is not well-known -- the SAT people offer a service not available in the "small test months". For an extra $15 (or was it $18?) you can have your daughter's ACTUAL questions and ACTUAL answers mailed to you along with her test scores. You sign up for this service /pay when you're registering her for the SAT. If you don't sign up when you register, I don't think you can add it on later. We did this with our oldest, and we used it as prep material for her second SAT date: We highlighted the questions she missed, and we found a pattern. She didn't need to review math at all, and her vocab and other grammar-type questions were top-notch. Like your daughter, she missed more reading comprehension than anything else -- she wasn't necessarily weak in it, she just wasn't as strong as she was in the other categories. So we found her practice in JUST those items, and when she took the test again her score went up something like 200 points.
People say that math-kids do better on the SAT, while English-history kids score better on the ACT. I don't know whether that's actually true, but my mathy-oldest knocked the top off of the SAT, and we didn't have her take the ACT. My English-history youngest just took the SAT today, so I can't comment on her score yet, but she is also taking the ACT later, just to have her bases covered.
Regardless, don't sweat it. A good student will be admitted to college, even if it's not the school of her dreams -- and not going to THE SCHOOL isn't that big a deal in the long run. At this point, I'd say it's more important to put effort into visiting schools and preparing for those applications that'll be in front of her on September 1st.