ive got unlocked stars for amount X.
i want one of those green stars but they seem far away.
is there any way to calculate how much i need to spent until this star is unlocked?
thank you!

In your star settings you have the information you looking for:

https://forums.d2jsp.org/settings.php?c=30

* Stars are unlocked by Purchasing more Forum Gold, or Adding Star Upgrades. All star amounts are confidential.

that was my question
so its totally random what i get

Not random; however, discussion of said stars' value in currency is prohibited.