Well first, no one said anything about a 650w PSU. You don't need that much power to run an RX 580.
Second, it took me all of three seconds on the Amazon UK store to find a decent power supply for much cheaper the 70 euro that would support an RX 580
https://www.amazon.co.uk/Corsair-VS...+power+supply&qid=1559856725&s=gateway&sr=8-3
There were other cheaper models like the EVGA W series but the VS550 has good reviews and is what I'd consider a baseline.
Off Topic.
Off Topic
Question, how many people actually have a PSU with below 500w capacity? I'm willing to bet the vast majority are pre-built systems or SFF (which is an entirely different conversation as size constraints come into play). So in essence "
pre builts or people with low budget 400w" are mostly the same thing. Unless you can prove to me there is this treasure trove of custom built desktop towers with 400w power supplies in them that people are using, your argument is essentially "This very minute part of the market should influence everyone's buying decision!".
The first half of my comment already proves you can very well get a 550W for much cheaper then you are stating here in the UK. Not that a single country's pricing would disprove my point as ultimately that is a small portion of the market. Mind you, electronics in general are more expensive in the UK and EU so by comparison to the increased cost of the overall build, the underpinning point I was making, in that the cost of a decent power supply is not much, still completely stands as the prices of the other components are also equally more expensive.