Q: if they have the same output voltage
A: The difference is in the quality of the components inside. There are different grades of resistor, transistors, even copper wire in better PSU's. Its like a car, a really cheap new car is not going to be anywhere near the quality of an expensive Bentley. They are both built the same way, but they use different quality of materials, and care in construction. - Usable power: a 720W generic psu for $25 will never, under any circumstances (with the possible exception of a lightning-strike) supply your machine with 720W of power. - Noise: a 720W generic psu for $25 will likely scream like a banshee. - Reliability: 720W generic psu for $25 has a good chance of dieing/shutting down if you try to pull anything like 720W from it for any length of time. - Accuracy: a 720W generic psu for $25 is unlikely to supply particularly accurate voltage levels, particularly when you try to draw significant current from it.