Let me make sure I understand the situation:
You made illustrations for a magazine (onetime use). You then resold the full rights to the images to a second company.
Logic might suggest you want to get at least an equal amount out of the art as you made from the first images (assuming similar time investment on your part)
Magazine price + buyout price. The justification is that this buyout is an exclusive, images have not been used by anyone else. Are you worried they will balk at the higher price?
Is that what is giving you problems, or is it the different style you mentioned?
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