First, Search the ProQuest Central database for Boston Globe articles on the Big Dig and its cost.
The State Library of Massachusetts has a searchable database of Big Dig documents online.
You will also find federal government cost reports on the Big Dig by searching the Government Accountability Office's (GAO) website (search: central artery and bechtel)..
See also the Mass. Inspector General's report: “A History of Central Artery/Tunnel Project Finances, 1994-2001.”
The Federal Highway Administration's (FHWA) Economic Development Highways Initiative has information on the economic impact of highway construction.