You have to look at the most rural and poor communities in the nation to understand why it can be both expensive and difficult. Birth certificates for instance, are often not present or available, nor are the agencies that provide the necessary documents accessible. Many times they are hundreds of miles from the person in need and the costs, and necessary steps can be extreme to a person with very little funds or resources.
To make them free enough that they don't impose a burden on people's right to vote, we'd probably need to collect an amount of identifying information in a central location rivaling anything attempted previously by the US government.
Enough biometrics to recognize a person regardless of whether they have any paperwork, paired with a system to update those biometrics when people's situations change (If you identify people by fingerprint for example you can't deny their right to vote if they lose both arms).
You should check out how DHS/TSA can do boarding pass checks now. Swipe your drivers license, passport, or DoD CAC smart card, all can auth in under 10 seconds, no boarding pass required. The data sources already exist.
I’d assume any digital credential system would accommodate biometric updates the same at my Global Entry interview went; you speak with a government rep in person when they take a digital representation of your biometrics.
I guess we can't escape the cost to implement that, even if that cost is subsided by the government. However, once it is in place there should be enough saving from increased efficiency to compensate that cost.