A power supply will drop its ability to supply power over time due to the capacitors aging and drying up. There is little difference in this regard between a expensive power supply and a cheap power supply. However a cheap power supply is very unlikely to be able to supply the power that it claims on the label. You can blame the Chinese for this as many unscrupulous Chinese companies buy relatively good cheap low power supplies and put labels on them claiming two or even three times the power rating that it can actually supply. They get away with it because most computers draw little over 100W and the true rating of a cheap 600W power supply is probably closer to 250W. This is why it is better to buy a bronze star power supply as at least you can trust that it can actually supply the specified power. The cheap power supply has less head room as it is working much closer to its maximum rating compared to a bronze star power supply and a reduction on its output power would have a much greater effect.