I personally wouldn't spend coupons on the Nine Tails Strengthen event.
With 700 coupons you won't be able to get full ninjas, so that's out of the question.
The average price for the cave keys in this event are about 18.75 coupons, which is more expensive than it's normal price of 13 coupons, obviously it's only an average value, but I still wouldn't spend on this event solely for the cave keys.
The medium-level refinement runes, you could get at a better value from the Missions Mobilization event at a value of 30 coupons per rune. While it would cost an average of 37.5 coupons in Nine Tails Strength, like I said before it's an average value so it can vary.
Similar to the cave keys, purification runes are more expensive in this event with an average value of 18.75 coupons.
Other than that, I doubt you'd spend coupons to get coins.
Also, the rewards for getting all the tailed beasts are mediocre and aren't worth the coupons and horrible RNG. Assuming you'd have to flip an entire grid to get all of the tailed beasts, it'd cost you 810 coupons.

TL;DR: The event isn't worth it.

The grid is 9 plates by 9 plates, so there would be a total of 81 plates per grid. Each flip costs 10 coupons/ingots, so it would cost 810 coupons/ingots to flip over an entire grid. The one free flip daily would reduce to cost to 800 coupons/ingots.

I'm confused as to how you managed to flip over the entire grid using solely 700 coupons.

Ah, I see. Thank you for the confirmation!