Return inflicts damage and has no secondary effect. The base power of
Return is dependent on the friendship of the user. The greater the
user's friendship, the greater the base power. The base power can be
calculated using the following formula:
Base Power = Friendship / 2.5
The base power of Return varies between 1 (minimum friendship) and 102
(maximum friendship). The base power is set to 1 if the formula
outputs 0.
Source: Return
Frustration inflicts damage and has no secondary effect. The base
power of Frustration is dependant on the user's friendship. The lower
the user's friendship is, the greater the base power of Frustration.
The base power can be calculated using the following formula:
Power = (255 - Friendship) / 2.5
The power of Frustration varies between 1 (maximum friendship) and 102
(minimum friendship). The base power is set to 1 if the formula
outputs 0.
Source: Frustration
Here is a list of all the normal moves so you can get a better idea of the varying strengths of Return/Frustration. I will highlight a few of the more common normal, physical moves to give you a better perspective of how much 102 base power actually represents.
Move Name Power
Explosion* 250
Self Destruct* 200
Giga Impact* 150
Double Edge* 120
Skull Bash* 100
Max Frustration 102
Max Return 102
Take Down* 90
Strength 80
Cut 50
* Has some sort of drawback (charge up time, recoil, death, etc)