Praise be to Allah.
We do not know of any saheeh evidence to indicate the amount of reward earned by the one who prepares the deceased for burial, but the Bounty of Allaah is vast. The one who prays (the funeral prayer) for the deceased will have one qeeraat of reward (a qeeraat is equivalent to the size of Mount Uhud), and the one who accompanies the funeral procession to the burial will have two qeeraats. For the one who seeks reward by preparing the body we hope that there will be even more reward.
Washing the deceased is fard kafaayah (i.e., if some members of the community undertake this duty, the rest are free from blame, but if no one does it, all will be held to blame), so there is no specific reward. But if sufficient people do the job, it becomes Sunnah for the rest. There are no reports which describe a specific reward, but there will be a reward for it because it is an obligatory duty, and not doing it is a sin.
Shaykh ‘Abd al-Kareem al-Khudayr.