What kinda works in determining whether there is a discount is to see if there is a regular discount for buying in bulk or quantity. If there are several prices then the group buy prices should be the same as the discount for buying the highest quantity.
BRS "Felt" filter sock 7 " x 16" 200 Micron (w/ plastic ring) - English
If you buy just one its 5.99, you buy 3 its 5.40 each and if you buy 5 it's 4.80 each. During the group buy this item would be 4.80 for ONE.
And the permanent discount that you get after you are in a group buy is worth a few bucks too. That same filter sock would be 4.80 for one when it's not group buy time. So you pretty much get the prices in the middle going forward.
I know ATI bulbs never go on sale anywhere but during the group buy they are a buck or two off each one. I bought 12 bulbs last group buy and saved over $20 so it does add up.