This is an interesting bit of calculation I saw on reddit, and something that NEM unfortunately will never have to worry about because of PoI and no wasteful mining requirements:
http://www.reddit.com/r/Bitcoin/comments/2rajx5/a_minimum_of_1470_bitcoins_has_to_be_dumped_on/

Just yet another advantage we will have over the vast majority of cryptocurrencies out there as time goes on.