Covering illegal immigrants in the full system would have to be paid by someone.  If you have 20 million people who need a very low estimate average of, say, $4,000 in medical care per year, that's $80 billion dollars per year additional, or around $1,000 additional per taxpayer per year.  

Personally, my medical costs average around $16,000 per year, including all doctors visits, prescription meds, dental visits, and eyeglasses.  And that's not particularly high -- it's not like I need surgeries or hospital stays or anything like that.  :)