My family and I flew from MAN to FCO with a ''low cost carrier'' last summer. Flights advertised at 39GBP one way and 49GBP the other seemed quite reasonable. Until I started to click the boxes! On top of the fares were purchase tax, airport duty, fuel surcharges, booking fee, card fee, check in fee (a bit rich as I did the on-line check in and printed my own boarding pass), baggage fees, seat fees and a meal. Total cost for four - 930GBP!
Some of this was optional - but the total was excessive in comparison to the originally advertised fare.