I think, much like when the ACA was implemented in the US, there's no real way to quantify beforehand how significant the changes will be (that's not saying we should do it and wait for the fallout). Yes, lettuce might go up to $20 a head--we'll eat less lettuce and it's no longer assumed to come free on burgers and people move to something else. We can offset that by offering farmer's more subsidies, importing more lettuce from somewhere else, or incentivize loans for better farm equipment/irrigation.

We'd need to decide to stop exploiting workers and work on finding solutions from there (I don't actually see that first part happening).