Is it possible to measure IO costs of a query in postgreSQL?

I have been trying for a week now without success to discover
if you can measure the cost of a query (with my c function).
EXPLAIN ANALYZE seems to give you the actual time it took it
to run but the "cost" seems to be a fixed estimate number and not
actual. I see in the code many times references to pgstats so
i figure there must be some kind of accounting.
Does anyone have a clue on this? its very important to me
to be able to measure scientifically the costs of a query.
10x.

Regards,
    tzahi.

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