Section 30. The commissioner, with the approval of the stewardship council may acquire for the commonwealth, by purchase or otherwise, and hold, or, with the approval of the governor and council, may take in fee by eminent domain under chapter seventy-nine, any woodland or land suitable for timber cultivation within the commonwealth or any land necessary for the erection and operation of forest fire observation towers. The average cost of land so purchased, other than land purchased for the erection and operation of forest fire observation towers, shall not exceed five dollars an acre.