Value is what the item is worth. For example, a $50 gift certificate from Walmart has a value of $50.
Let's assume you have a local flower shop and they give you a discount to buy a flower basket that has a value of $50, but you only have to pay them $30, then the cost is $30.
Value is what the bidder will see and determines how much an item is worth. Cost is basically your accounting tool to keep track of your expenses when getting items.