Helia has a food buff as well, but I'm pretty sure it's 60%. You'd have to take a moderate investment toward Dauros to get the buffs from Helia and Agrela, and depending on how much standing you need with Helia it might not be possible to get both at all.
Suppose you have all the food multiplier buildings, Fertility, and Harvest Blessing. That's +350% food output. Now let's look at the output of a size 10 city. You've got a pub and fishing village at +5 each, +2 from the city itself, and five farms for +15 total. Base food production is 27. With multipliers from buildings plus spells, food output is 121.5, or about 60.75 gold. Now subtract -12 gold for upkeep costs on your food multiplier buildings. There's three mana going into the spells, plus 120 mana for the initial casting cost. You end up getting about 49 gold per turn. Each farm you add on top of this gives you another 13.5 food, or 6.75 gold.
Now compare a gold based approach. A size ten city gets +2 from the city itself, +2 from the ratmen guild, +4 from the market, and +12 from four craftsmen districts, for a base gold production of 20. With multipliers from three buildings (+225%), total output is 65 gold. Each craftsmen district adds another 9.75 gold. You don't have to pay upkeep on any of this, so you end up making about 1/3 more gold by focusing on gold in a monster city compared to focusing on food in a monster city for a size ten city. Even if you've got a city with two pigs, or a Cheese Factory, or a Donkey, you're still not quite breaking even compared to a gold focused approach.
Just for the sake of comparison, a size 10 human city focused on gold produces 72.25 gold, and each craftsmen district brings in another 12.75 gold. Human towns produce ridiculous amounts of gold compared to other races, and as the cities get larger the lead gets proportionately bigger. If you factor in the +20% food or +20% gold mage perks, it gets even more lopsided in favor of producing gold instead of good, because the 20% bonus is additive rather than multiplicative with the other bonuses. Adding 20% more onto a 350% bonus doesn't make as big of a difference as adding 20% more onto a 225% bonus.