Passport cards are $65. But the cost is somewhat a red herring. There's nothing to keep the US from issuing a free voter identification card but issues of appropriate identification and the effort involved exist outside of out-of-pocket cost. [ADDED: And, of course, concerns about slippery slopes to national IDs which Europeans in particular may consider silly but nonetheless we're all allowed our hangups.]
And god forbid if you need a passport in two months or less--then you're looking at spending $200 to expedite the process (and add to that USPS 1-2 day document delivery).