The detriment caused by the breach of a covenant against encumbrances in a grant of an estate in real property, is deemed to be the amount which has been actually expended by the covenantee in extinguishment of either the principal or interest thereof; not exceeding in the former case a proportion of the price paid to the grantor, equivalent to the relative value, at the time of the grant, of the property affected by the breach, as compared with the whole; or, in the latter case, interest on a like amount.