Yamaha HP-50 for 15 euros
Audio-Technica ATH-2 + two Sony headphones + shipping for 10 euros
KWH HOK 80 mint for 15 euros, even if they have at times gone for half that
Pioneer SE-90D for 10 euros, I guess it's ok considering the rarity
AKG K 314 for 5 euros, bought seven pairs - not a headphone but still
vintage Beyer DT 990 for 47 euros was pretty decent
I guess the Beyer DT 100 for 5 euros was an ok deal in that people for some reason seem to regularly pay ten times that. But in terms of sound quality it was a huge loss.