various things...basis, your purchase points' "in and out" fee (what they sell the grain for over the amount they buy it for), etc.
For instance in my area (Texas Panhandle):
March Corn: 375.00, Avg Basis: +.10 = 3.85 per bushel. Elevator avg "in and out" fee: $.50 = cost for me to purchase: 4.35 per bushel
BTW...there is no Dec. Bean contract...you mean Jan.