I can tell you from owning a restaurant, the items with the best savings from our restaurant supplier over the grocery store are cream and butter. Heavy whipping cream is a good $1.50 less per 32 oz. carton from our supplier. Butter is routinely 50 cents to $1.00 cheaper than grocery store prices, per pound.
Surprisingly, cake mixes are routinely less at the grocery store (on sale). I asked our rep about this, and he said grocery store mixes have gotten so much better with pudding add ins and such, that they have surpassed what suppliers offer. His cake mixes start at $2 per pound, and you can often get store mixes in our area for $1 to 1.25 per pound on sale.