Section 30. Each register, except in Suffolk county, shall annually, before January first, file with the county commissioners an estimate in detail of the character, amount and cost of indexing which, in his judgment, may be required for the ensuing year, and the commissioners shall include in the estimates for county taxes so much thereof as in their judgment is necessary. The amount authorized may, under the direction of the register, be expended for the purposes specified in his said estimate, and shall be paid to the persons actually performing the work, on approval of the bills by the commissioners.