Decision with Scaling Prestige Cost

[ShadowNinja909]

I have a decision which converts Feudal vassals to Clan and I'd like the prestige cost to scale with the amount of feudal vassals. I've tried making a scripted effect but have no idea how to use it in the decision. The wiki says the values in cost = { } can be script values, but I'm not entirely sure what this means or how to implement it.

Here's the scripted effect if its needed:

feudal_to_clan_prestige_cost = {
    set_variable = {
        name = feudal_to_clan_cost
        value = 0
    }
    any_vassal = {
        limit = { government_has_flag = government_is_feudal }
        change_variable = {
            name = feudal_to_clan_cost
            add = 150
        }
    }
    remove_variable = feudal_to_clan_cost
}

I'm assuming the things under any_vassal work like a loop and iterate with each vassal, adding 150 to the variable each time. Let me know if this is not the case.