You make a fair point - there are plenty of free apps for Lemmy with no ads or purchases at all. For me personally, I use Sync (and paid for no ads) because I used it for years on Reddit and struggled to find a Lemmy app that worked/looked exactly how I wanted.

The price is on the high end, imo, but I don't consider it unreasonable given that I would've wanted to donate to the creator of whatever app I settled in anyway in order to support their work. The only difference to me is that this was a one-time upfront payment rather than a monthly donation on Patreon.