The thrid party candidate doesn’t need to win the election. Just disrupt it enough to allow the Dem to win. I don’t think it takes many resources. Look at how many votes the Ron Paul/Gary Johnson 3rd party plays in 2012. Enough to make a significant difference (despite Romney’s low appeal w Conservatives).
Gary Johnson received 1,275,923 votes (0.99% of the total vote) in 2012 on the Libertarian party ticket. He did not make a significant difference. And how many of these Libertarian votes would have gone to Romney or Obama? Would they have even voted at all if there was no Libertarian candidate?