heya, as someone who's bought and sold over 100 knights, ebay is the best bet on plastics, check ebay.com and ebay.co.uk (which tends to have better epic prices if they'll ship to you).
the plastic knights i've gotten were generally $3-4 per at the cheapest (in lots of 10 or so, not always in perfect condition). castellans and crusaders aren't too bad (about $8 per at the cheapest in lots of 3). The barons are a pain, you'll pay between 12 (if its mislabelled) to 20+ for one, and errants are 12-15 per at the low end too. wardens are harder to find in bulk, but they look goofy, so sometimes they're super cheap.
there are castellans, crusaders, and plastic knights on ebay now (not mine) if you want to watch them.
If you already know all this, sorry for wasting your time
--Aaron