I have done a handful of clinics, ranging from free/super inexpensive as part of a festival to a couple hundred dollars as part of a multi-day clinic.
All of them did a good job catering to each rider’s specific skill level. I had fun and learned something (or unlearned a bad habit) at each one.
I would say every clinic was money well spent. It “forced” me to focus on the basics instead of going straight to the fun skills (wheelies, manuals, jumps, etc). The multi-day clinics focused on the basic skills for the first half of the first day to make sure everyone had the same baseline. In the afternoon some progressed to other areas while the newer riders continued to hone their basic skills. The second day was out on the trail.
I’d recommend going to at least one, and then deciding from there if you would like to attend more. And I’d definitely recommend getting comfortable on flats before the clinic. You’ll get the most out of the clinic that way.