door lock problems

[BustaGwoove]

I've got this annoying problem with the driver's side door lock on my s13. First, when I lock it from the inside and close the door, the lock unlocks itself, so I have to lock the door manually from the outside with my key. Problem is though, that the lock is rusted or something, and turning the key to lock the door is a big hassle and it sometimes doesn't lock. Anyone know something I could do to fix this? Also, does anyone know how much a dealership would charge to replace the lock? Sorry if this is a repost, I searched and couldn't find anything. Thanks.