I see you're really good human wise, which'll probably help you charge more on Res just because... you draw humans. We like human artists here.
I think that for a digital sketch, you could probably charge 750k +, imo, and for traditional somewhere around 1 mil per., depending on how detailed it is. Somewhere around 2-3 mil for a coloured sketch/coloured digital? I'm not sure if you colour your traditional pieces but if it's nicely done you could get 3 mil +, I think. >>
Don't take my pricing as being very good though - I don't frequent the art forums as much as I used to. xD