The penalty should be death as vote fraud is a kind of coup. It is treason. It undermines the basic framework of the Republic and disenfranchises the citizen.
String up just one or two traitors and fraud will slow to a crawl.
I agree that penalties in vote fraud cases, if any penalty in given, are generally much too lenient, but I wouldn't go so far as the death penalty.
A couple of years in prison and a lifetime ban on voting in the future should serve as a deterrent. And, of course, a much larger percentage of voting fraud perps need to be prosecuted.
Punishment for voter fraud must be substantial. It is the worst form of corruption possible, because it creates so many victims.