Asking what a reasonable price is for a reg key and asking if users have older keys are hardly the same questions.
Nevertheless, there is only one authorized site to purchase Fab.
The developer is Fengtao and his homepage is DVDFab.com, which I believe you already have.
It is a hotlink on the cdf forum.
$50 seems more than fair to my mind, but as a prospective buyer, you will have to decide this for yourself.
Keep in mind that if you purchase, the BUY NOW button will link you to Fab.net which is where you'll actually purchase the software.
Do not download the non-css application from Fab.net.
The app that you currently have will work fine with your reg key.
Update only from Fab.com or right here from the stickies posted by Fengtao. The stickies will include Beta versions not available on Fab.com