If you have 50k pppoe users then you are in serious business. Mikrotik is great but you need something like
Cisco 6400 fully loaded wit all 7 modules which will include 32k PPPoE clients.
http://www.cisco.com/c/en/us/support/do ... aq.html#q5
even then, you will need two of them.
If You are doing serious business then you need serious investments so be careful with choosing equipment's.
I am convinced that your investments will be paid off.
Because 6400 is end of life, you should use some router like Cisco ASR1002 10G which can support 64,000 tunnels.
If one pppoe client make only 1$ profit par month you can return your investment in less then one month, for price ~30.000$.