Let's say that I have a city with a land area of 520. One farm will produce 20.8 food per day, currently worth on average $3,536 at a cost of $300 per day. This nets $3,236 per slot (since per slot is way more important than ROI, slots are very expensive). OK, now an aluminum mill, costs are $2,500 per day plus $2,652 worth of Bauxite ($884 PPU), and the gross income is $12,888. You net $7,736 per slot with the mill at current market conditions. In fact, unless your land value is 1,182 or higher, aluminum appears to win in per slot income.
As for your requested calculation, at land value 520, food gets $11.8 per dollar spent and aluminum gets $1.50 per dollar spent, but as previously discussed, the per slot income is much more important.