Upon a purchase or sale of a nonpurpose investment, gross proceeds of an issue are not allocated to a payment for that nonpurpose investment in an amount greater than, or to a receipt from that nonpurpose investment in an amount less than, the fair market value of the nonpurpose investment as of the purchase or sale date. For purposes of this paragraph (c) only, the fair market value of a nonpurpose investment is adjusted to take into account qualified administrative costs allocable to the investment.