He was doing a lot more than just living off it, Tweetie was the #1 Paid App for a long time and was in the Top 10 for all Paid Apps for months. He was selling thousands of copies per day for $2.99.
Just some math: $2.99 * 2500 copies/day * 30 days * 70% to LB = That is a whopping $156975 a month!