Discounted price not being updated when changing attribute (default price works fine).

If you have a product with attribute combinations and the product has a discount the "old" price does not get updated when changing attributes.

The "new" price works fine, but the old price (which should be updated) does not change.

The return data from shoppingcartcontroller is as follows:

return Json(new
            {
                gtin,
                mpn,
                sku,
                price,
                basepricepangv,
                stockAvailability,
                enabledattributemappingids = enabledAttributeMappingIds.ToArray(),
                disabledattributemappingids = disabledAttributeMappingIds.ToArray(),
                pictureFullSizeUrl,
                pictureDefaultSizeUrl,
                isFreeShipping,
                message = errors.Any() ? errors.ToArray() : null
            })

Which does not have a old discounted price. Is there a service method where i can get the "old price" which is discounted?