It's on the Mac App Store so you'll find the pricing there. Looks like $10 for one month (one time use maybe?), $20 for a year, $50 lifetime.
It’s a free app because you don’t have to buy it to run it. It will tell you how much space it can save you for free. So you don’t have to waste $20 to find out it only would’ve been 2kb.
But that means the parts you actually have to buy are in app purchases, which are always hidden on the store pages.