What we do know is that food is already very cheap in Africa. It's not obvious that food could get much cheaper, let alone the 300% price reduction that you give in your example.
On the other hand, we do know there are huge gains to be had when it comes to increasing income and economic activity in general. That's where our effort would be best spent.
Maybe I am not understanding the math properly. Food is cheap in the USA because we import them from poorer countries where labour is cheap. And we also consume a lot. This benefits the exporting country. But if we were to produce our own food, the local production won’t be cheaper.
Both of these feats would be incredible. So yeah, if we could do incredible things, that would be amazing.
Let's get back to reality though: Food prices are more or less set by international markets. African farmers can't compete with other countries, because they work so inefficiently. Making more people work as farmers doesn't change that.
That's the reason why Africans import so much food. It's actually cheaper. You don't need more people working as farmers, you would need automation to be competitive, but then less people are actually employed. Even then, this wouldn't lower food prices below market price, because those farmers can now export instead of serving the domestic market.
I do wonder if this accounts for SNAP and other food expenses not directly paid with cash income.
https://www.bls.gov/opub/reports/consumer-expenditures/2017/...
> The figures do not mean that food is more expensive in Nigeria than in the US. In fact, quite the reverse. The average American spends $2,392 per year on food, the average Nigerian half that: $1,132. The average Kenyan spends just $543 a year on food.
There are many benefits of Africa having the infrastructure/skills/industry to produce food in a more scalable way, but % of income is possibly not the metric to measure by.
[edit] changed last sentence.