The value of any asset consisting of an interest in property which -
(a) is determinable upon the death of any person or upon the happening of some other future event or at some future time or is a remainder, reversionary interest, right of fee subject to a life-rent or other future interest, whether vested or contingent, and
(b) is not at lease or reversionary interest expectant upon the determination of a lease,
shall be the amount which would reasonably be paid by way of consideration for an immediate transfer or assignment thereof.