Otonashi_Yuzuru
let's see.
Let's assume from the novel you read, 5 of them are premium, with around of 8 SS / chapter.
So we assume that once you read a premium novel, you just don't get any benefit from free chapter because you read it to the last chapter available. The average chapter / week for each novel, are about 14.
Now that takes us to the cost of (8x14x5)= 560 SS/week.
You got login bonus (~10), vote bonus (5x2 = 10), watch ad (~10), all in all you got around 30/day or 210/week.
Well, we take it that most of your spirit stone are exhausted by the 1st week. So with a 560 - 210 =350 SS/week or 1400 SS/month.
Now, for 1400 SS/month the best option (for now) should be the $10 for 1500 SS.
After 11 March, that would be $30 for the 1000+500 SS per month.
That is in under the assumption of you got at least 30 free SS/day, only read 5 premium novel with an average 8SS/chapter, and the bonus spirit stone did not expired before you spend it.
Now, my math might be wrong but, that's the money you will spend should you continue.