I remember to have taken flights for "2 Eurocents" within Europe. Cologne-Paris IIRC.
I can't remotely beat that! Back in the noughties, before APT reared it's horrible head, I took loads of impromptu day trips or weekends to varying destinations for between 5-20 euro all in. Hapag-Lloyd(HLX), Tui, EasyJet, even Ryanair to go to some ex-mil airfield in the middle of no-where, but close to my chosen destination. Even legacy carriers seemed to be doing it on short/medium haul routes. It couldn't last for ever and sadly for those of us who appreciate a good deal, it didn't. The most impressive aspect was just how long some of these companies were willing to operate routes at such low fares.