Dear Swaroop,
Thank your asking about something that probably is confusing to other people as well. Although we use "cost" as one way to remember "C", it is not the cost of the test (which is T). C is the cost (INCLUDING NOT JUST MONEY, BUT ALL OTHER BADNESS THAT MIGHT HAPPEN) of treating someone who does not have the disease. So C could be the "cost" of medication (including not just what it costs to buy it, but side effects rare adverse effects, contribution to antibiotic resistance, etc.) if the treatment is a medication, or the "cost" (money, pain, temporary disability) of an operation in someone who does not have appendicitis, if that would be the treatment.
(And by "treating" above, I mean doing whatever it is we are considering doing that the test is helping us decide about>)
If C were truly zero, we would always treat, and never need any test!
Tom