When the clock struck midnight, the Treasury Department’s much-maligned Treasury Asset Relief Program, or TARP, expired Monday. The $700 billion program was engineered to stave off an economic collapse, as banks — large and small — across the country started buckling from the weight of the mortgage crisis. For the past two years, its detractors have referred to TARP as a four-letter word and argued that it helps only Wall Street, not Main Street. But many of those who helped craft the program — from Congress to those within Treasury — maintain that TARP ultimately cost very little and was absolutely necessary at the time.