there's an easy way to do a very rough calculation.

look at how much cost things you know the price for sure and you have the reply.

cave key costs with no discounts 13 coupons each one.

advanced cloths costs with no discounts cost 63 coupons each one.

So we can consider that 5 keys cost 15 fortune coins, 1 advanced cloth costs 12 coins.

Means 1 coin = 5/6 coupons