/* set up the user */                                           
conn / as sysdba   
drop user jon cascade;
grant dba to jon identified by jon;
conn jon/jon
set serveroutput on;

/* repeatable code starts here */
/* clean up after the last run */
drop table t1 purge;

/* fixed length rows */
create table t1(c1 char(1000)) cache;

/* insert 20MB of data */
begin
for i in 1..20000 loop
insert into t1 values('aaa');
end loop;
commit;
end;
/

var y number;
var z number;
/* save the physical writes before truncate */
begin  
select value into :y from v$sysstat where name = 'physical write total bytes';
end;
/

/* do it */
truncate table t1;

/* save the physical writes after truncate */
begin   
select value into :z from v$sysstat where name = 'physical write total bytes';
end;
/

/* and the answer is, */
exec dbms_output.put_line('bytes written by truncate: '||(:z - :y))

/* repeat as necessary...*/