Real measurements are mostly better than calculations. Problem in this case is that most power meters are very inaccurate at low power consumptions (say, 50% error or more around 0.5/1W).
There is a nice comparison between different power meters: https://nl.hardware.info/reviews/1460/6/vergelijkingstest-9-energiemeters-tabel-met-testresulaten
Here you can see that some even cannot measure below 5W, others are very inaccurate.
I have a Brennenstuhl PM231E, which is not very expensive though quite accurate at low power (if not dead on accurate it’s at least an indication). I bought it at Conrad.
About the 100-300mA: that is the maximum drawn current @100-230VAC. So an indication for the efficiency at full load would be:
Input: 230VAC * 0.1A = 23W or 100VAC * 0.3A = 30W (lower input voltage might give a lower conversion efficiency).
Output: 5VDC * 2.3A = 11.5W
11.5W/23W gives around 50% efficiency… Not very good, though not too bad on full load either.
I did some measurements with the HKLM 230VAC-5/3.3VDC supplies, they are quite ok (also see reviews on the internet). Most switching supplies use up to 100mA when not used and efficiency is reasonable. Nothing to really worry about as long as the devices are useful
Currently I’m only buying well built supplies as I am more concerned with safety (fire/shock) than price. Added benefit is that both power factor and efficiency are mostly better as well.