Well...some of this thinner ones are a bit "bouncier"...but even using the bouncy ones helps train your core muscles even more. Head over to National Sports, or your local fitness retailer and have a look around. Check outhis website, or youtube vid on how to get the right size.
I have a quality ball, and a cheaper one..and they both get the job done. Although the cheaper ones are more prone to puncture from sharp objects. I think the most I ever paid for a Swiss Ball was $30, and the least was $5.99