Buildapc

Converting AC electricity into low voltage DC is fairly trivial. If you're so inclined, you can build a rudimentary 12v psu yourself for pocket change.

Maintaining a consistent power output within a certain tolerance under load is where things get complex and costly.

The finer your tolerance and more robust your protection systems, the more expensive your components get and the more of them you need.

Assuming that there is good market competition and prices aren't being jacked up for no reason, the difference between a $100 psu and a $500 psu is the difference between one with redundant protection systems, smooth and reliable power output, and a long life expectancy vs one that has far fewer of those things.

If you're slapping a replacement psu into an old system that you've had for five years, and you don't have a lot of investment in, then go cheap. If you're building new and high end, then don't cheap out on a component that could fry thousands of dollars worth of new equipment when things go wrong.