usage detection

Hi all
I have a mikrotik routerboard and I configured it like this
192.168.0.0-192.168.0.220 get internet from VPN connection
and 192.168.0.221-192.168.0.254 connects directly through ISP internet
internet connection is popeo connected by mikrotik

I want know exactly how much data my clients used (per IP or MAC)

I have one raspberry pi and also I have one openwork device

I read some tutorials about netflow
but as I understood (if I’m right) NetFlow is not free for raspberry pi for monitoring mikrotik

so I want know is noting is free for monitoring mikrotik by raspberry pi?
and is it possible that I use openwork for monitoring mikrotik by something link YAMon?
please help me
Best Regards