It's fair to say most of them ARE working. If most of them couldn't find employment, they'd be telling their friends back in Mexico to stay home.
How many work under the table? Therefore not paying taxes, and then their kids going to schools, using emergency room and other medical services, food stamps, getting arrested, doing time, all on taxpayer dime. And how much money is sent back to Mexico, enriching that country?
Have you read the figures of how many criminals incarcerated are illegal aliens? I can't remember but it's a heckuva lot. Costs taxpayers.