Sure, the quality and price and maybe even specialization will vary... and yes, of course, their services are valueable. (Thank you to all professional graphic artists, by the way!).
But there's one critical service that I don't see offered by any of them. And, yet, it's a valueable one.
And that's to provide basic consulting (or being an actual graphic DESIGNER), specifically if you are familiar with the niche.
What I mean by that is, I'm often left with the task of having to dig up and explain every single intricate detail of how I want my page to look. BUT, I'm not visually creative. So this means a lot of time invested into looking through my swap files and then more time devoted to a detailed email to explain it (I actually film a Camtasia video for my projects which has been appreciated by designers).
Still... I could just pay for someone to bring my design to life (that's an artist, not a designer) ... but I'd rather pay a premium to be able to bring someone in and say "here are 5 pages that I think work great, here's the message feel I'd like to convey visually, here's my target audience" and have that individual suggest everything from color schemes to imagery to fonts, layout etc.
THAT is worth premium pricing. It's similar to when you pull an interior decorator in the house. I could just buy the stuff and pay MOVERS to place them appropriately into the house... but what i need is someone who has seen a LOT of home designs and when I say "I want a modern-asian inspiration, with lots of open space, mostly dark metalic colors" etc they do the rest... they suggest paint colors, furniture, counter tops, little knick knacks etc.
But these days, it seeems all so-called graphic designers (again, all I'm finding is graphic ARTISTS) need every piece laid out before they move on a project. Not to mention, it's VERY RARE that I have an artist come back to me with an alternative suggestion.
YOU are the professional in this department (hopefully). So have a premium pricing added to your portfolio and invest some time looking at the most successful sites within your client's niche... (it helps if you also understand basics of what works from a marketing standpoint). But be willing to bring a detailed vision to your client vs. just putting together what he asks for.
And if YOU ever decide to offer this, make sure you contact me. Because if you do a good job as a graphic DESIGNER, I'll bring all the business from the current 7 graphic ARTISTS I have on my roster and bring them to YOU.