26 CFR § 20.2043-1 - Transfers for insufficient consideration.
(a) In general. The transfers, trusts, interests, rights or powers enumerated and described in sections 2035 through 2038 and section 2041 are not subject to the Federal estate tax if made, created, exercised, or relinquished in a transaction which constituted a bona fide sale for an adequate and full consideration in money or money's worth. To constitute a bona fide sale for an adequate and full consideration in money or money's worth, the transfer must have been made in good faith, and the price must have been an adequate and full equivalent reducible to a money value. If the price was less than such a consideration, only the excess of the fair market value of the property (as of the applicable valuation date) over the price received by the decedent is included in ascertaining the value of his gross estate.
(b) Marital rights and support obligations. For purposes of chapter 11, a relinquishment or promised relinquishment or dower, curtesy, or of a statutory estate created in lieu of dower or curtesy, or of other marital rights in the decedent's property or estate, is not to any extent a consideration in “money or money's worth.”