Before agricultural mechanisation, people used to spend 40% or more of their income on food. (the farm workers weren't really any better off then; they mostly got paid in lodging and farm produce instead of in cash)
We have to account for other improvements like better fertilizers, pesticides and the green revolution which would lower food prices independent of automation and migrant labour.
Also, the amount we spend on food isn't a 1:1 comparison because compared to the 1920s etc we buy much more 'processed food' (e.g. pre-peeled potatoes), which saves labor in the kitchen but is more expensive.