Determining replication traffic
On Apri 18, 2012, I addressed a similar question : MySQL : Does &#39;bytes_sent&#39; and &#39;bytes_received&#39; include mysqldump data?
I mentioned the following

Bytes_sent : IO Thread requesting binlog entries from the Master
Bytes_received : IO Thread reading binlogs entries from its Master
Bytes_received : SQL thread reading its own relay logs

You can find out what these are for all connections past and present by doing
SELECT * FROM information_schema.global_status
WHERE variable_name IN ('Bytes_received','Bytes_sent');

You can do so an individual session by doing
SELECT * FROM information_schema.session_status
WHERE variable_name IN ('Bytes_received','Bytes_sent');

I have good news and bad news. First ...
BAD NEWS
You cannot get a SQL thread to query information_schema.session_status. That's a violation of the first and fifth amendments (if applied against Replication's SQL thread).
GOOD NEWS (Sort of)
The only way you can track any kind of byte traffic is to do the following

Enable Binary Logging on the Slave

Dump all binlogs on the Slave using
mysqlbinlog --server-id=(server_id of the Master) ... &gt; Binlogs_From_Master.txt

Take the filesize of Binlogs_From_Master.txt and that's your best guess.


Subtract that filesize from whatever Bytes_received is, and that's the internal byte traffic on the Slave.
CAVEAT
You will have to set up replication to find that number out.
As an alternative, you could just use one the the following from the Master

Bytes_sent
Bytes_received
Bytes_sent + Bytes_received

as an educated guess.
Give it a Try !!!