Here is another balance board that helps you work on your core and improve your strength and flexibility. The Nalanda Balance Board Trainer can handle users up to 350 lbs. It comes with an adjustable block to accommodate beginners to more advanced users. The Nalanda balance board lets you try plank, squat, push up, and other moves with more difficulty.
You can get one of these for under $75 at this time. More info is available here.