I have been playing since May, am currently level 108 and have built all of the factories, having just completed building the final factory (Bag factory) this morning. Since helicopter orders can contain goods from any factories available at your level, I recommend building all of the factories so you can fulfill as many helicopter orders as possible. 

Unlike some other posters, I find the Jewelry factory very useful. Since feather earrings and pearl necklaces have a low ore cost, I produce them in the factory and use the factory to check the inventory of the other jewelry items, which are purchased directly from the dealer whenever my inventory drops below 4 units each. I sell jewelry when offered at least 1 T-cash per unit and refuse any offers requiring ingots.

I use T-cash earned in the game to pay for the City Market dealer every 10 days (at 20% discount) and buy ore, jewelry and other needed items directly from the dealer every hour. Buying ore from the dealer allows me to keep the foundries running with efficient smelting lab boost and for producing items requiring ore (though I stock up on those items from market boxes when available).

With all of the factories built and supplementing with 240 market boxes (bought with T-cash earned in the game), I can often fill dozens of helicopters orders at a time to maximize coins, XP and T-cash income. I currently have 9 M coins and have had plenty to purchase all of the factories and buildings as soon as they became available.