I think his point was, for 15 bucks you "commited" money so you will pay attention to him, if you like it chances are you will hire him to do more than just this 15 bucks worth of crap. Think of it as:

a) Building a portfolio: xyz sites use my designs therefore i rule.

b) Building relationships: xyz site uses my designs therefore i rule, which is why next time they need a design they will call me.

c) Show off: xyz site uses my designs therefore i rule and i will post it on my site, which is why people will see what I do any anytime people want a design i will be a known name and have a portfolio to show for it.

He'd do it for free, but nobody will take him seriously.

Now.

Where was I, oh yes, I will accept this 50 grand challenge. Submit proof that you are worth at least 10 grand.