Many German words for "cheap" are euphemisms. Why is that the case? The interpretation of these words just occurred to me, it might not be etymologically correct. I am not a linguist:
Preiswert = Es ist seinen Preis wert.
Günstig = Es hat meine Gunst.
Billig = Es ist mir billig. Ich billige das. (Im Sinne von: Es ist mir recht)
The words in their original meaning from today's perspective may be euphemisms but the meaning now is just the meaning now. People do not think of a connection to billigen when they hear billig. Same for günstig and preiswert which by the way is pretty neutral. There is nothing euphemistic about these words today.
There are subtle differences:
billig often means cheap in the sense that it is of bad quality. So you'll for instance say that "Das Gehäuse sieht billig aus" meaning that the casing looks cheap, bad. You also have it in terms like "Eine billige Anmache" (a cheap pass on someone).
When saying preiswert, I still mean it in the sense "It is worth it's price". I would not say "Das Gehäuse sieht preiswert aus" to say that it looks cheap. When I buy a "preiswert" computer, it means that it has a good price/value ratio.
günstig is neutral for me, but often refers to a good opportunity. Just imagine you want to buy yourself a new car which costs you 25.000 €, but you get it for 22.000 € because it is in green, which you don't mind, then it was "günstig".
