To be honest I think you selling yourself a bit cheap but I guess it depends on the time you spend on creating the work..
Maybe take what you believe to be a fair hourly rate for a job (not necessarily art) and apply it to the time it takes to produce what's asked of you..
I have no actual idea what the going rates are for this type of work but it's a fair logic to employ.
I checked over your deviantart page and there's some good stuff, I might have some work for you myself although I'll have to get in touch another time about it as I'm currently being attacked by my 1 & 5 year old kids LOL