True, there are many sources of information (some accurate, some not) out there on sound design that will cost you less than Syntorial. But they’re all missing the one thing that makes Syntorial so effective: two-way interaction. Books, articles and videos can only GIVE you information, and then it’s up to you to turn that new information into an actual usable skill. That’s like reading a medical textbook and then trying to do brain surgery. Best of luck to that patient!
Syntorial not only gives you the information, but it walks you through a step-by-step method in which you use your ear to program patch after patch after patch, and provides you with feedback at every turn to make sure you’re doing it right. You not only learn WHAT to do, you actually DO it, and BECOME a sound designer in the process.