That money has gone to service the National Debt, brought on by spending for social programs that began in the late 60s that was in large part the beginning of short-term borrowing that sent interest rates on their climb that peaked with Jimmy Carter in the mid 70s.
The result of short-term borrowing against high interest rates, combined with high taxation on a stagnant economy has produced an overall decline of which we can never recover unless we can insure another 20-30 years of lower taxation, combined with lower costs for goods and services.
Democrats want the reverse, because it is how they gain power over the electorate. Republicans have a responsibility to keep Democrats from regaining power, which is exactly why you see Republicans ready to sell you out on this issue. As my original point suggested, you cant blame immigration on immigration alone. It is just part of a much bigger issue.
So which group(s) represents the largest consumers of social spending (education, health, housing, etc)? We know who it is in Calif. The bottom line is that illegals do not add value to our ecomony at a sufficient enough rate to pay for services one expects of a 1st world country, hence the borrowing.
If we regularized them, then the combined cost of employer/employee taxes/workers comp/etc would be no different from any other low-wage citizen ie there would be no advantage to the employers hiring them.
Milton Freidman famously said that a society cannot both afford open immigration and a social welfare state. So there is a silver lining in all this: since nothing is going to be done about illegal immigration, we know the welfare state cannot last.