They're not cheaper than they were before. They're cheaper than they would be if there were no illegals to do those jobs.
Or do you think that without illegal workers, the construction industry would be able to find people willing to do such jobs for $8 an hour?
If illegals aren't an economic boon to employers, why do you think people hire them?
Housing and food are not cheaper. A house built by illegal labor is the same price as one not. Same goes for a meal.
The only beneficiary is the business owner. The savings in labor goes directly into his pocket.
If this is what you meant, then I agree - they are a boon to those who employ them.
What you implied was that we all benefit from them by paying less for what we consume. Unless I have need of a day laborer to clear my lot of weeds, there is precious little evidence that I, and the vast majority of Americans, am benefiting at all because of illegal labor.