All Tailoring items seem to sell for half the cost of the material used. The few builder and blacksmith items I've sold pan out the same way. Unless you already have a buyer lined up for the item expect to take a loss while leveling skills.
Food on the other hand breaks even or sometimes has profit. And a mill cow is more profitable then a milk cow. When fed 1 food (10 hunger points) can make 35 Flour or Cocoa Powder.
2 egg = 1 pet food = 500g
2 Wheat at 10g each per flour (700g worth of wheat to make 35 flour)
2 Cocoa Bean at 150g each per powder (10,500g worth of beans to make 35 powder)Flour:
1,200g in mats for the 35 flour that sells for 3500g. Cocoa Powder:
11,000g in mats for 35 powder that sells for 17,500g.
So the mill items are always worth it as long as you make sure to load up your mill until it says something along the lines of your pet will be starving before it finishes. And always prioritize cocoa beans if you have them, the NPC's give them out for their quests every 6 hours. You don't have to do only one thing you can line up any mix of items for the cow to mill. Its nice since you just set it up and it runs, you don't have to remember to run it again every 10 minutes like when making birdfood.
If you fed your mill cow to 100 it could make 350 items, I've never tried it so I don't know if there's a cap. You'd want to make sure you have the mats for that many first.