guaranteed profits, but how much?

[Rivky <rivky@xxxxxxxxxxxx>]

Hi everyone,

A friend of mine is setting up a subscription service with daily S&P 
daytrading signals. He really believes in his signals, and is planning to 
charge quite a lot for them ($500 or so a month!). Now, he'd very much like 
to offer a guarantee (if you don't make at least x amount, no charge), but 
would like to know what's an acceptable amount to guarantee...