As near as I can tell, the reason potions cost more than scrolls is because anyone can use a potion while only a spellcaster of the proper type can use a scroll, UMD aside. However, this advantage is not sufficient to justify potions costing twice as much as scrolls. My own inclination is to have potions only cost about 150% as much as a scroll, but this is the inclination of someone who merely wants to make potion use reasonable rather than encouraged. Since you want potions to be a staple part of your group's arsenal, you'll want to make them cost even less.