It's a service I offer that people are incapable of completing themselves (ie. getting an excellent grade on an English paper). As such, I can charge what I please and the customer can either choose to take that option or take the mark you would get if you did it yourself.
Keep in mind that $100 pays for my time, my skill, my demand, and my originality: no re-hashed essays here. Even if it were a horrible rip-off (which it isn't), it is the price a lazy piece of crap who doesn't attend class has to pay for the problem they've created.
Of course, you think that $20-$30 is "good money".....