do you think i have to charge more to release the whole rights?
It is called a 'buy out fee' as i understand and is usually an additional 50%
of the whole price. Otherwise the rights remain with you and you can do
what you want with the illustration... for example, sell it again for someone
else to use and on and on.
In certain circumstances you can release the rights for a set period then they are
returned to you ..... I am not overly sure how that works though