we stayed at Ty Mawr farm: walking distance from South stack, very friendly, ok prices and basic but clean facilities. It's the first farm on the right just after you leave the main road and drive up to south stack.
There's a campsite called Outdoor Alternative at Rhoscolyn. It's private and you need to ring and arrange to camp before you arrive.
And for North/South Stack we always used to stay at the Valley of the Rocks site. Not perfect but cheap and cheerful. There's a pub/bar adjacent to it, and I remember seeing a sign saying you could get a pint of Diamond White for a quid. Tells you about the type of pub it is (karaoke every other night, etc)! Never had any issues on the camp site though.