The deals on used electric vehicles right now are shocking. In August, used electric cars typically cost about $900 more than similar gas-powered ones, she added, which is the lowest gap on record. “There’s a really strong value proposition to buying a used EV,” Valdez Streaty said. But the federal credits go away at the end of month, which could cause electric car prices to inch upward. In the meantime, she said, the used car auction that Cox operates — Manheim — rates batteries on a scale of 1 to 100.