The detriment caused by the breach of a covenant against incumbrances in a grant of an estate in real property is deemed to be the amount which has been actually expended by the covenantee in extinguishing 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.