You may have noticed that DiskDigger is no longer as "free" as it used to be. This decision was a difficult one, and wasn't made lightly. When it came down to it, the freeware+donation model just wasn't sustainable anymore. So now, to get the full functionality of the program, you will need to purchase a license key for a very modest fee of $15.00, which is still a lot less than comparable utilities out there. I'm confident that this price is worth the vast improvements made in the new version of the program.
In a nutshell, the program is shifting more towards a "shareware" model. Here is exactly how it will work now:
Based on the above, you can technically recover all your files for free with DiskDigger. However, buying a license key makes it much more convenient, and is a great way to contribute to the project.