On a recent hop up to BOS, I heard a FA talking to a couple of the passengers in the row ahead of me about the credit card offer routine (BTW, on three out of the past four flights, I've noticed the announcement is muted in F class and piped through only to coach).
According to this FA, payouts are made on a scale basis based on the number of applications you sign up. The more you sign, the more you get. She made it clear that its not on a flat per-application basis. She also noted that one FA friend of hers who is apparently rather adept at the game made over $25,000 last year in signups.