$0, any direct fee is a barrier to a free and open process. And the local and state government already uses taxes collected to fund polling places and mail in ballots.
Things you'd have to make sure you've got covered (that the really don't have covered yet with the current system).
- Voter Identity and Authentication
- Multiple Ballots (you have to sign up for mail-in currently, and can't vote in person if you have)
- Coordinating these with the State and Federal election committees
If you want to see a mandate on non-polling place election methods, look no further than the special election here in Montana. It was put to vote to allow for all ballots to be mail in, and it was struck down via partisan reasons, even though those don't hold here. You would have to go through a vetting process and bidding period to be the person/group/company that supplied this tech, and someone would more than likely underbid you as federal contract rules are as arcane as they are inane.TL;DR: Voting is __not__ a system that is ripe for privatized 'disruption'.