The Government Accountability Office has released Tax Expenditures: Background and Evaluation Criteria and Questions (GAO-13-167SP):

Tax expenditures are reductions in a taxpayer's tax liability that
are the result of special exemptions and exclusions from taxation,
deductions, credits, deferrals of tax liability, or preferential tax
rates. Similar to spending programs, tax expenditures represent a
substantial federal commitment to a wide range of mission areas. If the
Department of the Treasury (Treasury) estimates are summed, an estimated
$1 trillion in revenue was forgone from the 173 tax expenditures
reported for fiscal year 2011. Tax expenditures are often aimed at
policy goals similar to those of federal spending programs. Existing tax
expenditures, for example, are intended to encourage economic
development in disadvantaged areas, finance postsecondary education, and
stimulate research and development. For some tax expenditures, forgone
revenue can be of the same magnitude or larger than related federal
spending for some mission areas. The revenue the federal government
forgoes from a tax expenditure reduces revenue available to fund other
federal activities, requires higher tax rates to raise any given amount
of revenue, increases the budget deficit, or reduces any budget surplus.

Given
the interest in tax expenditures' effectiveness, Congress asked GAO to
develop a framework that could be used to evaluate their performance. In
response, this guide describes criteria for assessing tax expenditures
and develops questions Congress can ask about a tax expenditure's
performance.