the only levels of tree that aren't worth the cave keys are 5 for certain as it gives 11 coupons or 1 key(buy 50 keys at once and they are 10.2 coupons a key) and arguably 4 since it gives 1 key for 9 coupons and some events give keys for less this one which gives 88 keys and 1 lvl 5 rune for 800 coupons which is 9.09 coupons a key but u also get the rune which adds better value.

lvl 1-3 dont offer keys so of course u take coupons.