Just saw on the white paper that Crowdsale participants get 50% of the tokens, so it looks like the investor break-even number is $12 billion now in payments =(
Edited my previous post.  

From their white paper:  
E-commerce   worldwide   sales   in   2014   were   USD   1.33   trillion   and   are   projected   to increase   to   USD   4.058   trillion   by   2020.   E-commerce   share   of   retail   sales    is   expected to   increase   from   7.4%   in   2015   to   14.6%   in   2020

Let's be generous and say worldwide sales are 3 trillion.   Monetha would have to process 0.4% of global payments for investors to break even in a year.