from what ive seen prices vary and theres about 4-5 tiers of pricing.

the cheapest ones can cost under 2k but they are also only 2star version ninjas(iruka was one of these for me and only cost about 1kish from treasures of the sage when i got it)

after that was the cheapest 3star ninjas where it was usually between 4-7kish coupons.

the next tier up pushes pricing to 10-12kish(ino summer is in this price range but depending on the event you might get cheaper as last weeks anniversary rocket event she cost from 6771 - 11695 Coupons/Ingots depending on luck)

after this theres the 14-20k prices(sosp is usually in this range and when we had the last fateful choice event sasuke[rinnesharingan] was also in this range)

and then theres the 20k+ prices.

if you join the discord under the event information channel usually they list events with coupon costs to get the ninjas(unless the ninjas on a wheel since thats completely up to luck to get) they also list costs to 4/5star them as well if you have the previous version already there if that matters to you.

do note usually if people are intenting to straight out buy a 15k+ ninja its usually best to aim for a fuku deal as well for bonus rewards but its up to you if you dont mind waiting longer and hoping.you c*so save coupons by getting as many free frags as you can during weeks you dont plan on spending on it.