How to quantify TEMPDB usage in relation to other databases hosted on same hardware?
Scott,

Check this statement out:

SELECT
    [Database] = DB_NAME ( qt.dbid ),
    [Execution Count] = SUM ( qs.execution_count ),
    [Total Execution Time] = SUM (qs.total_elapsed_time / 1000000.0 ),
    [Total CPU Consumption] = SUM ( qs.total_worker_time ),
    [Total Reads] = SUM ( qs.total_physical_reads ),
    [Total Writes] = SUM ( qs.total_logical_writes ),
    [Average Execution Time] = SUM ( qs.total_elapsed_time / qs.execution_count / 1000000.0 ),
    [Average CPU Consumption] = SUM ( qs.total_worker_time / qs.execution_count / 1000000.0 ),
    [Average Reads] = SUM ( qs.total_physical_reads / qs.execution_count ),
    [Average Writes] = SUM ( qs.total_logical_writes / qs.execution_count )
FROM sys.dm_exec_query_stats qs     
CROSS APPLY sys.dm_exec_sql_text ( qs.sql_handle ) qt     
LEFT JOIN sys.databases o ON qt.objectid = qt.dbid
GROUP BY qt.dbid


This should give you all of the information you need.  If all you're interested in is disk usage just keep the total reads/writes and average reads/writes.