Section 7. Said commission shall, annually in the month of February, estimate the amount of money required to pay the costs and expenses of the district for the following fiscal year, shall fix and determine the proportion of such costs and expenses to be paid by the constituent cities and towns thereof during such fiscal year which, however, shall not exceed any per capita limit established by the vote of two-thirds of the city councils and town meetings of member cities and towns, and shall certify the amount so determined for each city and town to the assessors thereof who shall include the sum in the tax levy of such fiscal year, provided that the per capita limit of any new member municipality shall not be less than the per capita cost to the member municipalities in the district at the time the new member joined the district. Such apportioned cost shall be on a per capita basis in direct proportion to the population of the city or town and the planning district as they appear in the most recent national census, exclusive of the inmates of county, state or federal institutions, and exclusive of the resident population of federal military installations to which the privilege of ex officio membership has been extended as provided in section four C; provided, however, that the national census or the annual national census estimate, whichever is the most recent, shall be used in any planning district in which the governing body of each city and town comprising such planning district accepts the provisions of this proviso. Upon order of the commission, the treasurer of each constituent municipality thereof shall, from time to time, subject to the provisions of section fifty-two and section fifty-six of chapter forty-one, pay to the district treasurer sums not exceeding the amount certified by the commission as the city’s or town’s share of the costs and expenses of the district. The commission is authorized to determine the amount of payment to be made to the district during the first year of membership of a new city or town in its respective district, but such payment shall not exceed any aforementioned per capita limits. Any such district planning commission established under the authority of this chapter is authorized to receive for its own uses and purposes any funds or moneys from any source, including grants, bequests, gifts or contributions made by the federal, state or municipal governments or by any individual, corporation or association. Any such district planning commission may contract with the federal or state government, or a city or town within such district, or with another district planning commission or planning council for the performance of planning studies and services within the limits of funds available to the district planning commission for such purposes. The treasurer of the district planning commission or, in his absence, the assistant treasurer shall disburse the moneys so received upon an order approved by the chairman of the commission when so authorized by a majority vote of said commission.