The satire here is that there is a bigger reward out for Jim's capture than for that of Huck's dad. Anyone who catches Jim gets $300 while anyone who catches Huck's dad only gets $200.
You can say this is satirical because Jim hasn't done anything wrong. All he has done is to try to become free. By contrast, people think, Huck's dad has killed his own son. You would think what he "did" is way worse than what Jim did. But because of slavery, the society's values are all screwed up. Twain is satirizing that fact.