hello,
the seleucid economy isn´t to bad. but you´ve got a wide country and you have a lot of enemies (= less or no trade) than others.
try to check your corruption. usually it´s about 15.000 mnai per turn. building mines and trade-buildings is a good strategy, but a slowly process. try to keep your borders straight or establish so many protectorats as possible, which bring you a lot of cash.