There are hereby authorized to be appropriated such sums as may be necessary to carry out the purposes of this subchapter, not to exceed, however, $104,500 for the acquisition of lands and not to exceed $5,102,000 (April 1970 prices) for development, plus or minus such amounts, if any, as may be justified by reason of ordinary fluctuations in construction costs as indicated by engineering cost indexes applicable to the types of construction involved herein. The sums authorized in this section shall be available for acquisition and development in the areas added by this subchapter.