If you want to spend coupons:

Cave keys (use them ONLY during rebate, and ONLY in multiples of 1050)> refines > charms > myoboku, summon cultivation > magatamas > purifications

Of course you should somewhat balance the features, lvl 9 charm won't do much for you with average magatamas and runestones. Lvl 10 refine won't do a lot with bad purifications, etc..

Why purification runes are at the last place? Because you will get them if you use cave keys anyway. Also they are quite common in events, so you will gather enough eventually.