It's a good general rule that if an artist is awesome, they probably do NOT have time for your requests, lol.
Awesome artists get paid for their work over at places like FA, DeviantArt, etc. Not many will do freebies, because the more awesome an artist is, most likely the more time they spend on a piece. An average burger-flipper at McD's gets what, $9 an hour or something? Some of these artists will spend 3-5 hours on a piece, so it is entirely understandable that they find it difficult to fork over $30-50 worth of work for nothing.
I find that people in general do not respect the artist's skill and time, not to mention all of the equipment they need. A good art tablet is at least $1.5k or more, if it's any serious artist. That's assuming they do digital art, and not physical art which over time could end up costing more over a long period of time.
It isn't cheap, and the artist spends a lot of time putting that stuff together. And, to top it all off, if an artist is good, then they are also popular, and have many requests/commissions/etc that they simply can't fill all of them.