I did a few donation acknowledgement letters. But it was very few. And I've seen the acknowledgement letters my parents received from the Corp for their generous donations to the mother cult *sigh*.
While WT is able to cash in on the JWs, I think it is actually a rather small amount in comparison with what other churches and charities are able to raise. Even a rather small Jewish group around where I live (sorry if this sounds stereotypical) is able to raise massive amounts of money that would make the GB faint out of sensual desire.
As far as JWs go, either they don't have money, or if they have money they realize that they'll get more fun out of spending the money on themselves before this wicked system ends.