In years past, if you wanted a 5V supply, to do it nicely you'd need seven components:
1. A transformer, ratio 240:3.6 approx, giving you 3.6V AC from the mains
2. Four diodes in bridge rectifier configuration, giving you full-wave rectified DC
3. A reservoir capacitor - pick a value - that smooths the FWR DC to a steady 5V.
4. An output fuse, to protect the charger (of no value to the thing being charged!). I'm assuming there would be one in the mains circuit, too - either a fused plug or a miniature circuit breaker.
You might complicate matters by adding a regulator, to ensure mains fluctuations didn't affect it and the circuit would need a slightly different transformer and a few extra things. The current available for charging would depend on the transformer. Sensibly, as the output would be floating, you'd tie one side to earth, so a fault would trip the RCD, too, and/or blow the fuse faster.
But the expensive bit of that is the copper in the transformer. Enter switched-mode power supplies. which I'm not going to explain here, except to say the idea allows for a much smaller transformer, is more efficient, but also potentially quite a lot more dangerous - to get to your 5V, high voltages and frequencies are usually involved.
Your "traditional" design doesn't emit any radio frequencies at all really, but a switched-mode supply has an oscillator, usually operating from around 20kHz upwards, so it is quite capable of being a radio. Furthermore the "switched" in the name creates nasty waveforms inside it, which also add to the radio-frequency interference it chucks out. They need suppression, and expensive testing of the designs to ensure they don't emit. This is often skimped. Then they "buzz", all over your radio reception, your WiFi, Bluetooth and so on, and even directly into your HiFi if the emissions are strong enough and the HiFi has issues.
It's a more complex circuit, they are often miniaturized but not necessarily reliably, they do consume power when not charging anything, if they fail you have to replace the entire socket, and, as has already been said, the circuit is in proximity to mains.
We have quite a few switched-mode units in the house which are either not earthed at all or inadequately earthed, including Laptop PSUs from leading brands. They are bigger, and supposed to pass stringent testing too, and they cost a lot more than these USB socket plates do. It's not unusual to get a belt from one.
So, of the socket plate, guess what proportion of its manufacturing cost goes on the USB charger. Given the retailer will probably be operating at 60% markup, and the OEM manufacturer probably makes 15-20% of their ex-factory price as profit, you can see this is the ultimate building down to a price for this sort of thing.
In summary, for me no bargepole could be long enough.
