Some more bits from that site:
"In Parliamentary written answers, Immigration Minister Mark Harper reported government expenditures of close to £28.4 million on deportation flights in 2010-11, an increase from £20.2 million in 2006-07. (See also Green 2010, Woolas 2009)."
And here's a bit of a breakdown of costs for you:
"Moreover, the flight itself is only one component of the cost of an enforced removal—the NAO’s 2005 report calculated the cost of removing failed asylum seekers at £11,000 per person. This total cost had the following component: £2800 for detection and arrest, £5800 for detention, £1500 for obtaining travel documents and “dealing with further representations” and £900 for removal itself. This means that the cost of flights (i.e. removal) make up only about 8% of the cost of enforced removals."
Costs of a non-forced deportation:
"A return through an assisted voluntary return program, on the other hand, cost about £1,100 in 2003-04 according to the NAOs 2005 calculation. Meanwhile, UKBA internal impact assessments estimate the average cost of removing a person directly from port at £500, or £1000 for cases that include overnight detention. Within the group of people refused entry and subsequently removed, it is not shown in existing data how many are removed after an overnight stay (or perhaps more than one night), and how many (if any) are removed the same day."