It's a couple Q's in on the page:

Q: How do I pay for Hyperspace? Hyperspace is a free download in the Mac App Store. Once downloaded, it’s free to scan an unlimited number of files. Scanning will let you know how much space is eligible to be reclaimed.

If you decide you want to reclaim that space, you will need to pay for Hyperspace using the purchase button that appears after a successful scan or by selecting “Purchase Hyperspace…” from the “Hyperspace” menu in the menu bar.