[munk-a]

> despite the fact that there's already a cheaper source

If there is already a cheaper source than growing Khat isn't more profitable. Khat is only more profitable if it can be sold for... more profit. That large profit should, assuming the market isn't distorted for other reasons[1], allow you to purchase more food than if you grew the food locally - if that isn't the case then growing food becomes more profitable.

1. I touched on this in a longer sibling comment, but a big reason is artificial supply restriction (as in, sanctions) with continued access to external markets for the black market goods.