most of the newer waves you can still get for retail prices pretty easily online. but if they have any series 1,2, or 3 for cheaper, I would snatch those up. they are the more rare ones. or the Darryl with Chopper. thats a newer one, but has gone up a little in value.
One nice thing I have found with Mcfarlane Walking Dead, is that they re-produce the earlier figures, maybe with a slight color change but no real difference. so you can for the most part get the earlier character molds, without having to pay an arm and a leg. Series 7 is going to be awesome with the updated series 1 Rick.