The cost isn't actually the main barrier to availability of ID. It's mostly the timing. They're typically only available at a DMV office, which are only open 9-5 on weekdays (or similar hours). This means taking a day off work, which for many poor people would be a major expense. Most won't have any form of paid time off, so they lose at least the days wages. Not to mention how few DMVs there are in many regions, and the resulting transportation expenses (need to take an express bus to get there in in time to wait in line and get the ID before they close).