Ah i dont think thats right;)
BeQuiet - 29€, efficiency at 50 % ->
86%
Seasonig G360 - 54,66 - >e at 50% ->
91%
So *if* they both had the same wattage then we could say that we have 5% efficiency discrepancy @50% Load (which is higher than you are usually), @20% its BQ85%/SS89% so 4%; 20% Load is ~60Watts so you will be >20 <50% usually.
So assuming you need 90 Watt, then the BQ will need 104,65 W to produce that and the Seasonic 101,12W
So thats 3,5W difference.
With 0,25€/KW and an uptime of 24/7 its €2,50/W/year -> so ~9€/Year savings -> After 3 years you'll get to a break even point.
The problematic point is to know in advance which load you will have and to find a PSU which peaks at your average (or even better: most frequently used) load.