Sync for Reddit had a low one time cost. But it was worth more to me than the few coffees I spent on it. I think the developer knew this, and upped the price to something reasonable. I was happy to pay, even though I already use AdAway. I'm a FOSS developer too, but there is good reason for an app being closed source (sole proprietorship and jury) and paid.

Also, my time is expensive. There's no reason the dev of Sync shouldn't also be paid.