Some of the resins are now back on lightning deal, I think the best deals are on the 2kg packs.

If you order two different 2kg packs that are both on lightning deal and have the buy 2 for 1 promo then the total is as cheap as $19.99 or $5/kg

Add any two to your cart, they must be different.  You can not get 4kg of translucent on the same purchase.

Translucent

Black

Grey

These 1kg bottles have good deals too, you can get any two different 1kg bottles for about $12 or $6/kg.  Exact final price depends a bit on which colors your get:

Black:
Grey:
Translucent:
Green:
White:

Here is the rest of the buy 2 for the price of 1 promos, the ones above produce the best deal.