The answer to this question can be found in chapter 14 of Lunch Money by Andrew Clements. Up to this point in the novel, Greg and Maura have been adversaries. In fact, Greg has considered Maura an adversary ever since they were toddlers, because she has a nasty habit of copying his ideas, making them better, and stealing his customers. The current rivalry is focused on the creating and selling of comic books. Greg is especially worried about Maura because he knows that her product is a quality version.
By chapter 14, Greg has received news that he and Maura will no longer be allowed to sell their comics at school. Greg figures it is now safe to tell Maura his honest opinion about her drawings. He is extremely complimentary and offers constructive criticism on how to make them even better. He then offers her a chance to have her work be a part of Chunky Comics. Greg offers her forty percent of the earnings. Maura counters with seventy-five percent, and Greg gives a counter of his own. Maura does not budge from her seventy-five percent, and she threatens to walk away from the deal and simply be a competitor. Greg agrees to the seventy-five percent because he doesn't want Maura as a competitor and knows that they aren't likely to sell anything anyway with the new school ban.