Operation Vaken, which took place between July 22 and August 22 in six London boroughs, saw mobile billboards take to the streets emblazoned with the slogan "Go home or face arrest".
Mr Harper said the 60 voluntary departures represented a "notional saving" of £830,000 based on the average £15,000 cost of an enforced removal.
The report said: "The average cost of a voluntary removal is £1,000, and the average cost of an enforced removal is £15,000.
"The 60 voluntary removals connected to Operation Vaken therefore represent a notional saving of approximately £830,000 compared to the costs of enforcing those removals.
"In addition, each person living illegally in the UK is estimated to cost up to a further £4,250 per year in costs to public services.
"The return of 60 individuals, as a result of Operation Vaken, may therefore have saved an additional estimated £255,000 in public service costs annually on this basis, with savings continuing into future years."
Last week Theresa May, the Home Secretary, admitted the vans were "too much of a blunt instrument" and will not be rolled out nationwide.
The Home Office report showed that of the 125 illegal immigrants who came forward two thirds were from India. In all, 90 Indians came forward, along with nine Pakistanis, six Brazilians and five South Africans.
In all, 15 said they had asked to go home voluntarily because of the ad van campaign. Another 19 said they came forward because they had seen coverage of the ad vans in media reports or on the internet.
The bulk of the remainder said they had seen other posters or leaflets which were distributed by the Home Office as part of the campaign, which cost just £9,740 in total.
So far 11 people have gone home specifically because of the ad vans.
Among those who contacted the team were a Nigerian man who had overstayed his visa by seven years and contacted the Home Office after seeing a report about the ad van on television. Another was a South African woman who came to Britain in 1999 and overstayed her visa.
The report also disclosed the advertising campaign, which encouraged illegal immigrants to send a text to the Home Office for information about returning home, received 1,561 texts of which more than 1,000 were hoax messages.
Britain's total illegal population is by its nature very difficult to measure but a report by the London School of Economics in 2009 gave an upper estimate of 863,000.