I've managed to book a oneway from SXF to BRU on FR for -8,96€*.
*The flight was 1,04€ via an OTA. A 10€ cashback promo on that specific OTA brought the price down.
For cheap flights, you'll have to pay attention to sales (e.g. the 1.300€ business class return to AKL on QR metal) and tipps. There's no universal formula for cheap flights.