Although I've not made direct measurements on these particular power supplies, they are all switchers. At moderate to heavy loads, switchers typically have efficiencies of 80-90%. Modern ones are not that much better. If the supply is delivering 100 watts, say, then it's pulling perhaps 120W out of the mains. If the supply were 100% efficient, at best you'd save 20W. Of course, real supplies aren't perfectly efficient, so let's split the difference and postulate that one might be able to save 10W by upgrading to a better power supply.

If you compute the savings over one year, assuming these numbers, and assuming 100% on-time, we're looking at about 90kWh saved each year (let's call it 100kWh). I don't know what electricity costs in your area, but let's say it's as high as 20cents/kWh (about twice the US national average). In that case, the annual savings is about $20 for this assumed scenario. And if the computer has any sort of a low power mode, or if the computer isn't on all the time, then the savings will be correspondingly diminished.