§ 13. Expenses. All officers, clerks, inspectors, experts and employees of the commission shall receive the compensation fixed by the chairman of the commission within the amounts appropriated therefor by the legislature. The commissioners and counsel to the commission may receive, in lieu of expenses, twenty-five dollars per day, and a proportionate amount thereof for each fraction of a day, while engaged in the performance of official duties at a place other than the city to which they are regularly assigned, and shall be reimbursed for the actual and necessary cost of transportation incurred by them; this provision, however, shall not be applicable to the chairman or a commissioner receiving a definite sum in lieu of traveling expenses. The officers, clerks, inspectors, experts and employees, shall have reimbursed to them all actual and necessary traveling and other expenses and disbursements incurred or made by them in the discharge of their official duties.