Essentially a minus in resources. (But can be regained depending on what you mill.)
Here are some Laval monsters you can send to the graveyard to trigger their effects: Laval Volcano Handmaiden can extend your milling by sending another copy of herself to the graveyard which can then also send another to the graveyard and so on. Laval Lakeside Lady can banish herself from the graveyard to destroy a set card your opponent controls. Laval Phlogis will boost the attack of all Laval monsters you control at time he was sent to the graveyard.