The only free classes are fab abs and stretch classes. These are usually scheduled between 7-8am. Have seen evening stretch classes at 5pm on some sailings.
All other classes eg yoga, Pilates, TRX, spin, boot camp are $20 per class, but they will sell a class pass which covers all of these for a set fee. This was $99, plus service charge for the last cruise I was on which was a 14 day.
Suggest you go visit the fitness center once you board - they’ll have the schedule for the cruise available so you can see what they offer and decide what you want to do.