tax preparation services that do not charge unless you save?

I got the idea from law firms that take your lawsuit, but don't charge fees unless you win. I was thinking it would be cool if there were any tax preparation services out there that do not charge you money unless they save you more money. For example, you fill out your tax form with turbotax or whatever, then hand it over to the company. The company would probably also interview you as well to uncover any additional areas. If they can save you more taxes, then you have to pay them a certain percentage of the additional savings? Are there any out there? I would think it would be pretty lucrative, the companies could charge 50%-75% of the additional savings. Most clients who would never have used a tax preparer otherwise would be willing to do it because there's nothing to lose.