Execution Plan - Quick Question

Hi guys,
When you view an estimated or actual execution plan using SQL Server
Management Stuido you get a number of cost values for each of the
operations. One thing I've never been sure of is what unit these costs
are measured in.
So for example, one of my queries has a CPU Cost of 3.XXXXXXXXX and an
IO cost of 8.XXXXXXXX. Most of my queries have a cost much much lower
than this.
I used to believe that these values in particluar represented "seconds"
but that would mean my query would take around the 11 second mark, which
it definately doesnt.
Can anyone tell me what these numbers actually mean in the real world?
Many thanks
Simon
I read in "Inside SQL Server" that this is the # secs to run on a test
machine in the MS test labs.
Cheers,
Paul Ibison SQL Server MVP, www.replicationanswers.com .