Trying to think off the top of my head of "public domain" books I've paid for:
- The Aeneid, translated by Robert Fagles
- Various Dostoevsky works translated by Pevear and Volokhonsky
- The Master and Margarita translated by Pevear and Volokhonsky
- Dead Souls by Nikolai Gogol, translated by...you guessed it
- El Seņor Presidente, translated by Penguin Classics
- The New Annotated H.P. Lovecraft
Then, yes, the answer is overwhelmingly for a quality translation.
"Additional value" by way of footnotes, forewords, etc is nice, too.
Might be slightly tangential to the topic, but one possible option could also be "Wanted a
consolidated list of works when not all works are yet in public domain." I know personally I've bought a "Complete H.P. Lovecraft" or "Complete Conan the Barbarian" collections in the past, because I didn't feel like being confined to only the pieces that have entered US public domain yet.