I do agree using an image of Roger Federer is copyright infringement, but if the OP used a photo of a package of Wilson strings, with a photo of Fed on the packaging it would not.
As for price for stringing. It all depends on where you are located. Here in So Cal. the going labor rate for a Syn Gut String Job, is $10, but in the San Francisco Area it's closer to $20. I've heard in NYC is closer to $30 labor. I think the cost of labor should be what people are willing to pay for, reasonably. So if you are the only stringer for 50 miles around, you can justify $20 labor.
A majority of people looking for a stringer are recreational players. So it's a good idea to stock up on a couple reels of Syn Gut, like Gosen OGSM. I go through more reels of Gosen OGSM than any other reel of string I normally have.
Thats because the string hes promoteing their product.