If you are a casual p2p player like me who only pay 10 bucks / month you will get :

+1800 cp from monthly (300 instant + 1500 in 30 days)

+1000 cp from that extra ss plunder for 20 days (10 bucks give you 500 ingots, 300 for monthly, 200 for plunder)

+2550 cp from convoy/plunder assuming you never fail (85 cp/day for 30 days)

+250 cp from monthly sign-in.

+300 cp from online pack (10 cp/day)

+450 cp from wishing tree assuming you have lv 7 tree which required around 1 and half month playing to reach (15 cp/day)

+400 cp from 9 tails assuming you get 25 cp every time.

+640 cp from group summon assuming you get 40 cp every time (16 times total in 30 days/1 month)

Total 7390 coupon per month just from consistent daily activities and some cash. Not even count extra cp you get from weekly events like myoboku trial which give free 60 cp/day or weekend event like puzzle.
Premium ninjas cost around 4k to 20k max (from sailor sakura to some op op ninjas that not even release yet). So you only need around 3 months of saving for the most expensive one. That's a reasonable time imo. I don't count ultra rare like itachi and hashirama edo or other ingots exclusive ninjas tho.

Go for ninjas or power is depend on your situation, whether you comfort with sage, matsuri or not. No one know that better than yourself.