Hey,

The most worth way to get Itachi Edo, is either a good enough synergy with Fukurokumarus Deals, while having a free to play friendly event as well. There is not really a ,, Cheapest“ way, but ,,most worth“ way, as Itachi‘s price has been around 20-22k or so. If you are a spender, definitely the recharging rebates which may have him are a very good option. Another awesome option is the lucky board. For around 20k investing in there you‘ll get awesome items for your power. Consider from the other events to gather some free fragments of him instead.