purpose of customerEnteredPrice

Hi there ive been amending the productvarientsingrid.ascx to add a feature where the customer can design a product to their specification before adding it to the basket. there are some additional features that add to the total price much like how a product variant attribute would work. I use this method to add the product to the basket after they have created their product:
decimal customerEnteredPrice = Convert.ToDecimal(hdnTot.Value);
this.ShoppingCartService.AddToCart(
                            ShoppingCartTypeEnum.ShoppingCart,
                            pv.ProductVariantId,
                            attributes,
                            customerEnteredPrice,
                            quantity);


i thought customerEnteredPrice would just set the price shown in the basket but it doesnt. i dont know what its purpose is? how can i make sure the total price is the value of my hiddenfield?

thanks
phil