In most servers 400$ per month would've been enough, yes, especially if you invest into Froggies (1st and 4th one; what we have this week) every month.

Tips, huh...

Always prioritize redeeming cave keys from grocery store and use them during cave key rebate to get an ultimate essence (you get a guaranteed essence every 1050 cave keys).

Don't buy basic/medium refines and basic/medium/advanced cloth. You will acquire them eventually.

Don't buy basic/medium/advanced threads, unless you are swimming in money ;) It is the worst power feature to invest into

Clothing (NOT cloth) is a good way to increase power, try to collect as many outfits as you can (though it's kinda hard to keep up with the rate OASIS keeps releasing them).

Save your charms, refines, threads and cave keys for rebates, so you will get more value out of them.

Consider leaving your refinement level at 6-7 and save over 1200 adv. refines to upgrade it to level 10 during rebate (you will save 300-350 adv. refines).

Stack your position one ninja, focus on one set of equipment. You can't make all of your ninjas strong easily, but if you focus on one ninja, it is doable (stack charms, refinement, runestones, attack and ninjutsu magatamas).

Unless you feel really lucky, don't invest into RNG events (without redeemable section), it's a very easy way to waste money...

Madara [Founder] is obtainable from drawing in GNW treasure (you have a chance to get him or Kurama Naruto at 900-1200 scrolls spent there, if you get Kurama, you'll need to draw another 900-1200). Madara [5 kage summit] and Shisui are event only ninjas.

Event ninjas are cheapest in events such as Treasure of the Sage and Time limited recruitment (also Sakura's Gift used to fall into this category). But personally I don't like spending coupons/ingots in these events, because you only get what you pay for. Events such as Lucky board and Great tree also have event ninjas and you will get a lot of power by paying slightly more for a ninja you want.