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 (modulated by rnd). Succubi do not gain stat XP from seduction in and of itself.