So I was trying to decide if buying the Quoin Multiplier cards were worth it or not, well I found the potential "break even point" for a variety of scenarios.
Each scenario assumes that you will collect you daily limit of quoins, and that you are ONLY collecting iMG quoins with a base value of 1 while your mood is "FULL".
If you collect 100 quoins a day you will be able to buy a 1.0x multiplier card and earn ALL the iMG spent on it back from quoins in a single game day when you reach a multiplier of 317x.
If you collect 150 quoins a day you will be able to buy a 1.0x multiplier card and earn ALL the iMG spent on it back from quoins in a single game day when you reach a multiplier of 211x.
If you collect 200 quoins a day you will be able to buy a 1.0x multiplier card and earn ALL the iMG spent on it back from quoins in a single game day when you reach a multiplier of 158x!
Of course you will most certainly start "breaking even" sooner with all of the iMG that you gain through various game actions. Also, if you are lucky enough to nab some iMG quoins in the Ancestral Lands then you can back the iMG much quicker.
But those are merely the bare minimum quoin collecting break even points. However, until you reach those levels of multipliers it is an uphill battle, but after you pass those marks your iMG will begin to soar and each subsequent multiplier card will be easier to buy.
So in closing, the cards are very much worth it if you plan on making the investment and taking your multiplier to those levels, otherwise your iMG may be better spent on other things.