Sadly, there isn't a reliable range of boxes needed to spend on a ninja in boxes. You see posts of people getting ninjas with a few boxes to people after spending 10k coupons/ingots then they complain that they didn't get a ninja. Don't spend on mission boxes or lucky board just for the purpose of getting the ninja unless you are prepared to spend lots of coupons/ingots and receive nothing.

Well the thing for hashirama and tendo is that you can get the via treasure but it takes time and it's 50% chance for hashirama or tendo at around 400 seals. Other ninjas like minato hokage is more of a trophy for collectors rather than be a reliable ninja on your team due to his lack of sturdiness. I would rather know I'm spending my coupons on something I'm going to get then leave it to chance.