I'm so sick of this excuse. The correct response is "... so let's not do that this time, mkay?" Specifically, make them absolutely free-as-in-beer and only require minimal time and effort to obtain (benchmark: proof of address is too much effort). You do this explicitly on the basis that voting is a universal right and requiring money for it is abhorrent, as is any other obstacle not strictly required for ensuring uniqueness.
Now you can also get rid of the half-assed national ID system comprised by our driver's licenses, state IDs, etc. And the people who want to use voter IDs to disenfranchise people have to shut up because you've addressed their supposed point.
Get it over with, already.