Etherchannel STP cost calculation

Hi ,
I am trying to find out how the cost for etherchannel STP is calculated .
Please see the below output. Please help me understand how the cost has been
calculated as 3 for PO4.
Thank you,
ddcxs01a-ddcxra#sh etherchannel 4 sumFlags: D - down P - bundled in
port-channel I - stand-alone s - suspended H - Hot-standby (LACP
only) R - Layer3 S - Layer2 U - in use f - failed to
allocate aggregator
        M - not in use, minimum links not met u - unsuitable for
bundling w - waiting to be aggregatedNumber of channel-groups in use:
19Number of aggregators: 19
Group Port-channel Protocol
 Ports------+-------------+-----------+--------------------------------------
-------4 Po4(SU) PAgP Gi2/13(P) Gi3/13(P) Gi4/13(P)
 Gi5/13(P)

ddcxs01a-ddcxra#sh spanning-tree vlan 27
VLAN0027 Spanning tree enabled protocol rstp Root ID Priority 27
      Address 0009.112f.b040 Cost 5 Port
     1668 (Port-channel4) Hello Time 2 sec Max Age 20 sec
 Forward Delay 15 sec
  Bridge ID Priority 32795 (priority 32768 sys-id-ext 27)
Address 000b.60f7.e180 Hello Time 2 sec Max Age 20 sec
 Forward Delay 15 sec Aging Time 300
Interface Role Sts Cost Prio.Nbr Type---------------- ---- ---
--------- -------- --------------------------------Po4 Root FWD 3
        128.1668 P2p

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Received on Sun Aug 22 2010 - 06:58:01 ART