It's in the init, but it doesn't work. It's supposed to be tied to the arrival of the Tax Collector, but immigrant nobles never arrive, and even if you mod the entity raws to make them manually assignable, they don't do the special jobs they're supposed to. So, the Economy never gets enabled.
The problem with the way the Economy worked was that there was no modeling of supply and demand whatsoever. The price of housing was fixed by the value of the room, even if no dwarf on the map could afford to stay there, so it was easy to have a fortress full of homeless dwarves and empty rooms. The price of food was also fixed, and if you went in for any kind of high-value prepared meal production it was easy to have stockpiles of masterwork roasts sitting uneaten while all your dwarves gnawed cold plump helmets. It also didn't help that individual dwarves had no clue whatsoever how to make intelligent purchases, and would randomly opt to buy silk loincloths from the last goblin invasion and then get evicted for not paying rent. Admittedly, that last one accurately reflects some people's real-world purchasing habits.
The point is that in DF the main result of activating the economy was that most of your dwarves would be unhappy and many of the usual happiness-increasing measures (nice rooms, fine goods, masterwork roasts) would become unavailable to them.