Arachin 20 - Real vs Symbolic Value
Sometimes symbolic value of a person is stricter than the real. For example, if he vowed his symbolic value to the Temple but died before paying, his heirs must give it: since the symbolic obligation is fixed by the Torah, it exists from the moment he vows. By contrast, if he promised his real value (as defined by being sold on the slave market), and died before he was appraised, his heirs need not give anything, since dead have no monetary value.
If one vows to give "half of his symbolic value," he gives half, but if he vows to give "symbolic value of half of him" - then he pays the full amount, because half of him cannot live without the other half. In this respect, real value vows are the same: if he vows value of half of him, it is tantamount to the whole value.
If one promised his ox as a sacrifice or his house as donation to the Temple, and the ox died or the house fell, he does not have to pay. However, if he promises their value, he will have to pay if they are not extant.
Art: Childe Hassam - Back of the Old House