Jeez, some people are really pissed about the cost of removing ads. The developer needs to eat like anyone else, and he either gets the money from Google tracking you, or from a payment to hide those ads. Considering that the userbase isn't very large, it IMO is a fair price for never seeing an ad again in that app, the dev will not collect a penny more from your usage at this point.
We've been pampered by mobile apps being dirt cheap for so long, and the dev's track record with Sync for Reddit IMO speaks enough that he's going to support it for a long time.