Section 14. The financial liability, referred to in section eleven, of each city and town for the suppression of public nuisances named in section eleven shall be determined annually by the commissioner, who shall notify the city manager or town manager in cities and towns having such a manager, otherwise the mayor in cities and the selectmen in towns, on or before January first of his determination. The determination of liability shall be based upon the last preceding valuation of the city or town made for the purpose of apportioning the state tax, and shall set forth an amount for the control of the Dutch elm disease and the beetles which spread said disease, and an amount for the suppression or control of the other public nuisances named in section eleven, the total of which shall not exceed in any year one fifteenth of one per cent of said valuation. In determining the said financial liability, the commissioner shall consider the degree of infestation or infection, the area involved, the location of the municipality in relation to its neighbors, the number and specie of public and private shade trees and the location and size of woodland and swampland areas therein.