Dubai is paying ~$2,450 per acre-foot of desalinated water. You generally need around 2 acre feet of water per acre of farmland assuming near zero rain, it varies by crop type but goes up with temperature and down with humidity.

Farms growing food crops don’t produce ~5,000$ in profits per acre, even 1/10th that is an extreme outlier. On top of this desalinated water still has significantly more salt than rainwater which eventually causes issues. Subsides can always make things look cheaper when you ignore the subsidy.