Firstly, I never charge for my work, as I just adore to see people happy, and for someone to want/request my work in the first place is honour enough and a great challenge and experience, so I feel no need to charge as the payment for me is the buzz i get from doing it.

(Plus I just simply LOVE it here at HM, and like others have said would just be mega happy to help out my fave site for free anyway! you know..give a little back for all the fun and awesome art the site and its members have given me.)
Anyway...back on topic... just out of interest and curiosity after reading this thread, I trauled the net looking to compare prices of various 3d artists ( of poser etc )

... and I found that if you are a top ranked artist you would generally seem to ask for ?5 - ?10 per digital image, or if commisioned to do a print that is shipped nationaly it's roughly ?15 - ?20 (average)
( BTW ... 1 GBP = roughly 2 USD
2 AUD
2 CAD
1.5 EUR
Just for those of you wondering what the conversion rate from ? to $ is. :) )
I know that for my own "2d/graphite" tattoo creations last year I was once offered ?10 per flash sheet. (which i turned down for the same reasons as above. :) )
Sorry, I have no clue what a "Manip" artist would charge, I'm sure some of the other members could enlighten us on that one.
I hope this helps a little.