Here is some quick math for you, lets suppose you have a 500 W power supply, a system that pulls 250 W at full load, and is on 8 hour a day running at full load all the time(unrealistic but makes for much easier math), that puts your PSU at 50% load so its going to be at peak efficiency.
Power drawn from the wall:
Basic - 312.5 W
Bronze - 304.9 W
Silver - 294.1 W
Gold - 277.8 W
Platinum - 271.7 W
That means that over basic you save 7.6 W/h with bronze, 18.4 W/h with silver, 34.7 W/h with gold, and 40.8 W/h with platinum. With 8 hours a day at full load that comes out to be 1.82 kWh, 4.42 kWh, 8.33 kWh, and 9.79 kWh saved per month with bronze, silver, gold, and platinum respectively. If you pay 15 cents per kWh that means that over the course of 1 year having a platinum rated PSU over basic saves you about $17.62 so it could take quite a while to pay for the difference with energy savings alone, but in general silver rated units and up are much better quality than basic rated units.