1) Open borders even under the assumption of no welfare state (total free markets) is a bad idea in this day and age. This has to do with the time it takes to socialize and assimilate immigrants successfully. It took decades to assimilate Italians and Irishmen properly, and it gave the US severe problems (rampant corruption cultures in our biggest cities, the mob, etc.). It will take longer to assimilate Hispanics, given their more alien backgrounds, poorer educational attainment, etc. etc.
2) Labor costs as a portion of total costs in agriculture are minuscule. Hourly wages would have to be something like USD100 an hour before they became prohibitive. Just to give an example: I worked at a vineyard last year. We harvested a ton of grapes per man in four hours, for which we were paid 100 dollars per ton. A ton of grapes makes 750 bottles of wine. That's 7.5 cents per bottle of wine.
You could pay three times that, and the consumer would not notice.
If we deported all illegals - and we more or less have to - the labor market for unskilled workers would shrivel. Business would have to get workers from other industries. What would it take to get an American college kid to work in San Fernando Valley field for eight hours? A lot. And it should. Back-breaking work in a field needs to pay enough to let you retire early, for obvious reasons. Why is this feasible? Agricultural productivity has skyrocketed the past decades. Yet none of that productivity gain has manifested itself as higher wages for manual laborers. If you could make USD40 an hour without a high-school education, we might even be able to find something for the people of Detroit to do.
A key to reviving the American economy is thus dealing with this issue.
Other important steps should be tax-simplification (abolish all credits and deductions, cut marginal rates, raise taxes on capital income, normalize payroll tax, abolishing the corporate tax, scaling back federal regulation, shut down agencies, and converting Federal welfare programs to direct cash transfers.