Elin:Race/Succubus/talk

Here are the relevant code snippets from the game:

Determination of stamina cost:

            if (chara.IsPCParty || chara2.IsPCParty)
            {
                chara.stamina.Mod(-5 - EClass.rnd(chara.stamina.max / 10 + (succubus ? StaminaCost(chara2, chara) : 0) + 1));
                chara2.stamina.Mod(-5 - EClass.rnd(chara2.stamina.max / 20 + (succubus ? StaminaCost(chara, chara2) : 0) + 1));
            }

SP cost is determined by difference in MaxSP of both parties. Determination of stat to be boosted:

            foreach (Element item in tg.elements.ListBestAttributes())
            {
                if (c.elements.ValueWithoutLink(item.id) < item.ValueWithoutLink)
                {
                    c.elements.ModTempPotential(item.id, 1 + EClass.rnd(item.ValueWithoutLink - c.elements.ValueWithoutLink(item.id) / 5 + 1));
                    c.Say("succubus_exp", c, item.Name.ToLower());
                    break;
                }
            }

And the potential is determined by the stat which has the highest base value, value being the difference divided by 5, plus 1. Succubi do not gain stat XP from seduction in and of itself.