I think that the subtext of the question is fascinating.  Certainly, Medea is fairly manipulative and self- serving in how she assists Jason to escape from his challenging predicaments, even killing her own brother. Yet, I think that Jason is equally driven by self- interest himself.  His desire to marry Creon's daughter is not done to restore some sense of global justice nor out of altruistic means.  He does so to further his own self- interest, in the same manner that he was able to use Medea to feed his own self- interests in his time of danger.  The fact that he lies to Medea when confronted about his interest in Creusa reflects that he has little problem in exercising the powers of manipulation for his own benefit.  In trying to extricate himself from Medea, Jason is quite adroit in manipulating the situation to a "blame the victim" scenario, where he manipulates the situation to make Creon's banishment of Medea her own fault and then engages in revisionist history in minimizing her role in attaining the Golden Fleece:

He [Jason] never accepts responsibility for his new love alliance. Medea has a valid complaint, yet Jason attributes her anger to a' 'stubborn temper" and blames her banishment on her inability to submit to Creon's will. Jason is made even less sympathetic when he minimizes Medea's role in helping him obtain the golden fleece (a feat that involved killing her own brother so that Jason could escape) and suggesting that Medea is merely jealous and not legitimately hurt. Jason almost deserves the punishment Medea serves him.

Medea is fairly manipulative, but Jason is not without blame and the manner in which he is depicted reflects that he is just as guilty of reflecting manipulation and a sense of cunning deceit as much, if not more, than Medea.

This is a great motif to consider in this play.  Medea manipulates everyone around her in order to get what she wants from life.  That Medea has to use manipulation is an interesting point through.  Does this mean that she doesn't have any power of her own without the use of manipulation?  Does this suggest that women, in general, use their powers of manipulation to get what they want becuase they won't be treated as equals with men?  You will have to decide that for yourself after reveiwing the play, but you could consider the following examples of manipulation:

1.  Medea's first scene expresses her hurt and grief over her lost marriage to Jason.  She is angered that everything she did for Jason has been tossed aside.  The chorus first enters to basically tell her that "men will be men" and that she just has to accept what has happened.  But by the end of the next scene when she explicitely states how women are treated as lessor citizens of a relationship she has completely manipulated the chorus (of women in this play) to be on her side and cheer her on as she states that she is going to make Jason pay for what he has done to her and her family.

2.  She manipulates Creon into having just one more day to "get her affairs together" before leaving Corinth.  He even tells her directly that he doesn't trust her and that he regards her as a danger to his daughter (Jason's new wife), but she still sways him with a bit guilt and appeal to family.

3. She manipuluates Aegeus into feeling sorry for when she tells the story of Jason to him.  She also uses her abilities with magic to manipulate his cooperation in providing a safe harbor for her in exchange for her help with his and his wife's fertility issues.

4.  Jason arrives to try to justify his choice to marry the princess as a means to secure his family's position in Corinth, and at first, Medea will have nothing to do with his "logic." But later, she talks to him again and claims to completely agree with him.  She is manipulating him in order to allow their children to bring a present to the princess, their new step-mother.  Medea fully plans to have that present, a beautiful gown, actually be a poisoned garment that will destroy the princess. 

5. She even manipulates or justifies to herself the killing of her own children.  She is certainly doing it because it will be the most horrific punishment she can inflict on Jason, but she also tells herself that she would rather kill them herself rather than have them killed by her enemies who will certainly come after her now that she has killed the princess and the king of Corinth.

Medea is clearly a master of manipulation and ultimately accomplishes all of her goals -- no matter how horrific the results.