This question is difficult to answer because it is not clear exactly what is meant by "cost of port X" on a given bridge.
Let me use your latest topology to introduce STP in a nutshell:
A BDPU contains the following parameters, considered in sequence when it's a matter of comparing them:
-1- Root ID,
-2- Root path cost,
-3- Sender Bridge ID,
-4- Sender Port ID.
That's why the terminology of the IEEE specs calls the content of this message a message priority vector. The cost is just part of the information.
A bridge perform a root election taking into consideration their local priority and the best information (BPDU) they receive on any of their port. Here bridge A has the best Root ID in the network (meaning the lowest bridge ID). It means that it will beat any BPDU it receives and stays the root bridge.
Bridge A, the root, sends BPDUs with a Root path cost of 0. B will receive two different BPDUs from A, on two different ports. It will have to elect a single of those ports as its root port. To do so, B uses a "Root path priority vector" computed from the ?message priority vector? received on 0/0 and 0/1. I'm not going to insist too much on the IEEE terminology, because it's probably ignored by the CCNA exam you are preparing;-). You can just assume that, for the purpose of the election of the root port, the (locally configured) cost of the port receiving the BPDU is added to the root path cost received in the BPDU. So here, both port 0/0 and 0/1 have a root path cost of 0+19=19. The determination of the root port will thus be made using additional parameters in the received BPDUs. The two BPDUs have the same root ID, root path cost and sender bridge ID (they both come from A), so the sender port ID will make the difference. Here, 0/0 is likely to become the root port of B (because with default configuration on A port 0/0 will have a lower port ID than 0/1 on most Cisco bridges).
Bridge B has determined its cost to the root from the best BPDU it received. So its root path cost is 19, and B will include this value in the BPDUs it sends out to C on 0/2 and 0/3. Let's run the algorithm another time before I confuse you;-) Bridge C receives two BPDUs with the same cost 19 on 0/0 and 0/1. As they are coming from the same bridge, bridge C will use the sender port ID to elect (probably) 0/0 as its root port. The root path cost for bridge C is 38, this is the value that will be included in the BPDUs sent by C.
Now, suppose that I am configuring a cost of say 1000 on 0/1 on bridge C. The root port election will still designate 0/0 as a root port (because port 0/0 leads to a root path cost of 19, while port 0/1 leads to a root path cost of 1000+19=1019). But you can already feel how ambiguous the term "cost" is. When you talk about the port cost of 0/1, do you mean:
-a- The configured cost (1000),
-b- The cost received in the BPDU (19),
-c- The root path cost used for the root port election (1019),
-d- The root path cost of the bridge, i.e. the cost that the bridge would advertise on this port if it was designated (38)?
Also, 0/1 is a port that is actually receiving BPDUs and that is considered for the root port election. If the port is designated, it only sending BPDUS, and you cannot compute -b- or -c-.
I hope that it becomes clear why it's not obvious to answer: what is the cost of this port on this bridge;-)
