Most players have quoted 900 - 1200 seal scrolls for super rare in Kage. I think you have spent about 300? So that's another 600 at least and 900 at most. You need to think about whether you are prepared to continue spending up to 900 seal scrolls and ignoring any new treasures that come out.

You can make a rough estimate of how much time you would take to farm 900 seal scrolls as well.

1) GNW 30 to 40 a month.

2) Monthly event 8 a month.

3) Matsuri lets give an average of 5 a month (2 times weekly)

4) Daily Login rewards 2 a month

5) Insert how many seals you are buying a month with $

I can't think of anywhere else you can get free seal scrolls, so that's about 45-50 a month for f2p. You will need about 18 to 20 months to farm 900 seal scrolls.

That's about 1.5 years, which by then we might even have 4.0 with all the op ninjas.

However, we live in the present so it's debatable whether to save or spend and the decision is entirely up to you. I think the number of scrolls you have already invested into kage will impact on the decision because its how much nearer or farther you are from the super rare.