Normally:
Avatar 100x100px = 5-10K, depending on specifications
Signature (reasonable size) - 5-20K, depending on specifications.
Banners etc etc for websites and the like (like signatures, but bigger) - 10-30K, depending on specifications.
Wallpapers (all standard desktop resolutions) - 20-70K.
Renders (price varies a LOT, depending on image, so I'm not going to give a guideline, just show me the image and we can negotiate
Drawings: These are a lot more expensive, pretty much because they take a lot more time and effort. Black and white: 100K+, colour: 400K+ (obviously, this is negotiable, as it won't cost nearly as much just to draw your character's face or something.
Now, seeing as though I've just started out doing this service, the first 10 people to post here get their's done absolutely free.
Preferable layout
Type of graphic you want:
Images you want in it (Max of 1 for avatar, 3 for signatures, 5 for banners and 7 for wallpapers):
Colour scheme:
Desktop resolution (if applicable):
Additional information:
Thankyou <3
Remember, these do not have to include Guild Wars screenshots etc etc, though they can if you want.
Lets see that creativity shine 
;-)