It's been suggested many times over the years. Since WGT promotes itself as a "realistic golf game" they could join the real world by putting a discount on balls purchased by dozens or more. When you walk into your local pro shop, golf retailer or mass merchandiser golf balls are almost always sold by the dozen (or more). Those boxes are always sold at a discount from the 4 x individual sleeve price.
Beware of what you ask for, though. WGT may very well start selling dozens for exactly the same "per ball" price the sleeves are currently priced at and raise the prices of individual sleeves.
As for a monthly sale, I'm reminded of many of the "sales" I see promoted around here. Companies simply raise the price and then discount to the current price as a "sale". The end result is that they raise the base price of everything.