Elections are expensive anyway. You either need to maintain thousands of computers or print millions of ballots (assuming nation wide elections in most countries). But the main cost is in the organization. You need to rent it places for the polling stations, set up the booths and the whole infrastructure around it. Security and people checking ID’s. Some of this will be done by volunteers in most countries, but that won’t cover all the costs. In the grand scheme of things I don’t thing electronic vs paper voting matters that much to the total cost of the elections.