I've never bought a fully-flexible ticket myself - but did have one for an external review - it was business from BHX-FCO (back in the old days when BA flew somewhere useful!). The cost was 1500UKP (at the same time a 'cheap' business ticket was 120UKP); the meeting finished early and upon arrival at FCO the check-in staff put me on the next flight leaving in 20 minutes (HBO) - so useful for the flexibility!