More Americans than ever are getting college diplomas, and it’s costing them. Adjusted for inflation, tuition has increased more than 50 percent since 1999, spurring students to take on crushing loads of debt. It’s set to get worse on July 1 when, barring a congressional deal, the rate charged on federally subsidized Stafford loans will double from 3.4 percent to 6.8 percent. The change means bigger payments for families ill-equipped to handle them. Over half of all student debt is held by households whose net worth is under $8,500.