Monthly card is the way to go if you plan to do about 5$ a month, since you turn 300 ingots into 1800 coupons and like 98% of things in this game can be bought with coupons for same rate is ingots. Do not use ingots on anything, just slowly turn them into coupons for much better value.

Dice event is very good, maybe the best one in terms of how much stuff you get per coupon however at start of game i would recommend the medium refines from it rather then ninjas.