bigger lots mean no set up fees which mean less cost per pound.
bigger lots mean less transportation cost.
$2.31 is insanely high priced. when was the last time you shopped your business around?
Formosa is by far the cheapest raw resin supplier I know of. they lead the charge down in pricing 3 or 4 months in a row now in CDI. Oxy and Georgia Gulf seem too eager to raise prices early month only to have Formosa cut prices around the 20th of the month retroactive to the first. PVC in the states is in the low 50's now for nearly any grade. asian resin is about .381 per pound if you can get it here cost effectively.
you are buying pellets from a massive compounder in small lots. try a small custom compounder and you will likely get a better price. i no longer work there, but http://www.polyflexcompounding.com/
has very good pricing on small lots. set up fee is reasonable for small orders there. a guy like polyone laughs at your volume. go somewhere where your business is needed and you will get a better price.
i don't know what additives you have in that compound but i doubt they are paying more than 60 or 70 cents a pound for materials cost.