There are hereby authorized to be appropriated such sums as may be necessary to carry out the provisions of this subchapter, but not more than $12,125,000 for the acquisition of lands and interests in lands and not more than $92,813,000 (July, 1971 prices) for development of the recreation area, plus or minus such amounts, if any, as may be justified by reason of ordinary fluctuations in the construction costs as indicated by engineering cost indices applicable to the type of construction involved herein.