Hypothesize why the reward for Pap is only $200 and the reward for Jim is $300 given that Pap is suspected as Huck's killer.

The fact that finding Jim is worth more money than finding Pap is a reflection on the values of southern society during the time.  To people in the south finding a runaway slave more important than finding a man who may be a murderer.  Jim may be innocent of anything except for wanting to have his own life, but he is more "dangerous" to the south than Pap who actually killed his own son.