The first Supply Center built will give anywhere from 100 - 150 resources depending on the upgrades you have invested prior to the battle. Each additional Supply Center will give 30% less resources per minute compared to the previous one.

Say for example you have 1 maxed out supply center, it will produce 150 resources a minute. Your next supply center may only produce 120 resources a minute because of a present percent decrease. Your next supply center will produce 96 resources a minutes, then 76.8, and so on. Keep in mind this is an example, and in this example the percent decrease for each additional supply center is 20%.

The Supply Center is an investment to many, having multiple supply centers in the beginning of the battle allows for rapid base expansion later but slows down your initial base development in turn. While having only a couple of Supply Centers may seem like a bad idea, those resources that are saved instead of building another Supply Center, can be used to produce units in order to form a rush attack. So it's all a matter of choice between aggressive gameplay and a more economy focused build.