Here is my .02, opinion only, and hopefully not tooo inflammatory:
What is the value of a good relationship with a loyal customer base?
Basically, the group buy isn't as profitable as selling stuff at 200% cost. I agree.
But...
nobody can expect to sell 100 buckets of salt, or 25 PH monitors, or whatever the group buy is, either way. It isn't like a huge profit potential was lost (aka you could have done the same volume at higher prices). By hosting a group buy, a bunch of people show up. Sure, you make a smaller profit on the goods, but if 20 people now know where you are located, stop by to pick something up, and develop a relationship, can you put a value on that group buy?
While they are there, they see you've got good prices and they like you, after they pick up the few impulse buys (cha ching), maybe they take a card. Even if you don't make a single dime on the group buy (which would be poor planning), how much sales does the added visitor traffic bring? With the right attitude, store, and planning, you could be recieving families, friends, and extensive contacts from the internet of just one person who you sold a 50 dollar item to. Are GB's still not worthwhile?