Tornado balls are great for building explosive power and core strength. They are not the most expensive tools in the world, however, it does get expensive if you need to keep increasing the weight by buying a new one. In order to solve this issue, I recommend building your own tornado ball. I tried various tools such as cargo bungee nets in order to build a tornado balls and the best thing I found to be helpful is a basketball net.
All you needs is a couple carabiners, duct tape, rope and of course a medicine ball. This is pretty self explanatory and I think you can easily figure this out. There are "how to" videos out there if you need some additional assistance.
I like to use medicine balls since they bounce back and I can keep a high frequency/rhythm during my longer workouts with them. A basketball filled with sand will also do the trick and this can enable you to change the weight through out your progression.
I hope these tips are beneficial for you and you can reap the core strength and explosive power benefits of the tornado ball.