The prices of prayer potions are high due to the amount of people demanding them. Although seeds are fairly easy to gather, same goes with herbs. But ranarr seeds aren't dropped by many good monsters, nechyrael and dagannoths, which are common slayer tasks. As well, the amount of areas you can use prayer potions is wide. Most of the monster hunting, barrows, fight caves and more. Super restores cost much, because you can't get snapdragons as monster drops, which makes it a bit harder to get. The demand is high, just because they are very effective while using saradomin brews and they also play the same role as prayer potions do.