Multilateration traffic is heavily dependent on the ADS-B / Mode S ratio in your area and how much overlap there is between receivers. More Mode S or more overlap means more multilateration candidates and more traffic.
It’s somewhere around 3-5kB/s for a busy site in Europe with good overlap:
06/14/2015 16:14:45 mlat: Receiver: 960.3 msg/s received 18.3kB/s from receiver
06/14/2015 16:14:45 mlat: Server: 0.0 kB/s from server 0.0kB/s TCP to server 4.6kB/s UDP to server