The answer is 40m~50m~~~~!!!
I have played this star force game for so long, countless of times that all my equips reached 19 stars. Yes, 40m~50m is the bare minimum cost you need to get your equip to 15 stars. Some may take 700m~1b, some may even take 2b or 3b to get to 15 stars.
This is because if you are unlucky you will be up-ing star and down-ing star at 13 and 14 stars. And for several times, you will be back to 10 stars again. And again, you will encounter the same situation that you just can't reach 15 stars. It keep failing at 14 stars....this is the reason why maplers can spend up to 3b and cannot get to 15 stars at all..
Nevertheless, the cheapest you can get to 15 stars is about 40m-50m... I know a lot of people have been asking this question... a lot of them want to know how much they need to get to 15 stars. There you goes, that is my answer for you.
