I've always found staff travel to be very cheap. £5 flat fare plus tax. It means even Tenerife and Paphos can be £35 return if both sectors are on staff travel fares.
The problem is of course that the seats get released onto staff sale using historical data (I.e last years sales) and so there are times when the day before the flight departs only 90% of seats are booked and it still doesn't release staff fares because the 'the computer' thinks it will sell those remaining seats. This isn't usually the case and combined with no shows some flights are departing with a fair few spare seats that could otherwise be filled by fare paying staff fares. Albeit very cheap fares.
This is where standby staff travel will help when it gets rolled out at the end of the year.