That's not really the whole story is it, thirdfromleft.
Fiscal studies are one thing, but there are other issues, such as employment, pressures on housing, schooling, and so on.
People in sectors facing competition from immigration (pretty much all low-paid jobs, plus IT, and others) will tell you that immigration depresses wages and reduces job opportunities.
Often immigrants doing low-paid work are objectively better at the job than native workers, because the immigrants may be more motivated, better qualified, more highly skilled, yet still happy with NMW as it would be a lot of money where they came from. Obviously that is good news for employers, but it's not great for those on the dole.