The immigration monster and the "go home" vans.
Only, as the reporting of Mark Harper's written answer makes clear, it costs more to enforce a deportation (£15,000) than the average illegal immigrant costs the taxpayer a year (just shy of £5,000). The latter figure seems difficult to believe, in any case: most illegal migrants won't/can't access public services, and so will use hardly any resources at all. The motivation behind the campaign is then somewhat financially sound: paying for a flight for someone is hell of a lot cheaper than doling out money to our friends at G4S or Serco to "Mubenga" someone.
The problem was in the execution, but then that was clearly the point. This was a stunt straight out of the Lynton Crosby playbook. Wait until news was slow, then launch a campaign using a borderline racist slogan designed to attract both condemnation and attention in equal measure. If some people did take up the kind offer, all the better. The Tories could portray themselves as tough as well as practical, and Labour would be caught in the trap of either condemning sending illegal immigrants home, or condoning a 70s style National Front demand. They didn't however factor in that this being the social networking age, a thousand people would prank the phone and text line, or indeed that even Nigel Farage would denounce the campaign as being too nasty, designed purely to win back some of those who had defected to his party.
Without figures for voluntary deportations for a similar period prior to "Operation Vaken", we clearly can't make a comparison as to how successful the whole charade really was. It might well be that a similar number to the 125 total claimed to have been motivated by the operation would have submitted themselves anyway without prompting. This is the thing: there is absolutely nothing wrong with ensuring those here illegally know they can return to their country of origin if they so wish, with the government picking up the tab. It's how you go about doing so, and telling people to go home or face arrest is manifestly not the right way, not least when it's clearly a political campaign designed to look tough and win votes. It probably does save money, although the idea the Vaken might have saved the taxpayer £830,000 is ridiculous.
Something that wouldn't just save money but actually benefit both the taxpayer and the economy would be an amnesty, bringing those working cash in hand out of the shadows and onto the path towards citizenship. That however would go completely against the rhetoric and policies of the past few years, where politicians have followed public opinion rather than attempt to lead it. Too bad that as Sunny wrote previously, it's now probably too late: the monster is loose.