The tax database and voter database are already intertwined in many countries (it's how they know where to send voting cards to), it would on a operations-management level be relatively simple to checklist "anyone who is a net ower of taxes before the taxes already deducted step".

Personally, I think voting should cost one dollar. One dollar is nothing. Many people won't bother picking up a dollar off the ground if it is slightly out of the way, leaving it for a kid or beggar to find instead. A fountain drink costs more than a dollar in most places. If the future of your country isn't worth one dollar, you shouldn't be interacting with the future of your country. It's small enough that objectively speaking it doesn't matter, but exists enough that it would filter out all the slacktivists and leeches.