It goes down by 100 people. Which even if they are paying £100 for their ticket the industry loses £10,000. Or roughly 1/6th the cost of employing one driver for a year.
I was just using small easy to compare numbers, in reality the Cornish branch lines have much higher numbers:
Passengers made 169,388 journeys on the Tamar Valley Line between April 2010 and March 2011, an increase of 21.8% on the numbers seen during the same period in 2007 and 2008. This gave the line ninth place in the branch line league table. On the Looe Valley Line, 95,446 journeys were made between April 2010 and March 2011, an increase of 23.8% over 2007/8, giving the line eighth place in the table.
An estimated 945,000 journeys have been made on the Maritime Line since the improved service was introduced on 18 May 2009. This is almost two thirds more than the 579,081 journeys made in the previous two years. In 2010, 477,500 journeys were made on the line, more than two and half times the 185,000 recorded in 2001, the earliest year for which figures are available.