You pay 10% (ID90) or 50%(ID50) of the full fare before tax or surcharges. The full amount of tax (ie based on the full fare) and surcharges is then added to what you pay. For the UK this is by far the most expensive part of the ticket.
Note these are standby tickets- there is no certainty of getting on any particular flight if it is full of commercial passengers. I see you are in Bangalore. Flights to and from India are often overbooked and depart full.
Other than that it is a good way to see the world if you are flexible about travel dates.