Guys,
I can tell you that all hero's star's are "fixed" At the end you will wind up spending the same amount of stars for each of them. Some fail consecutive in a row for like 30 - 50 times.. and then you will get the next 3 stars in 5 more tries.

Stick with it. You will get it to stick eventually.

As for the star amount, it will vary slightly, but the rule of thumb.

Blue to 9 = 500 - 800 stars

Purple to 18 = 500 - 800 stars

Good luck