You should spend on keys, even if directly, if your runes are lv 4 or lower

You should wait for key packs and never buy individual keys (i.e. pay coupon to explore) if your runes are mix of 4s and 5s

You should not spend for keys except maybe the key mystery fruit if your keys are mix of 5 and 6

You should not spend on keys once your runes are all at least lv 6. (FYI, 200 key at this point will give you ~200 power @lv 5 charms, and this is during rebate)

Mind you, you should pick key over coupon in the wishing tree regardless.

For clothing, 60k is the bare minimal, I'd wait for ~70k.

Otherwise, coupon should go to lucky board.