Dynamlically updating the estimated cost of a transaction

Hi,

I am looking for a way to let the user know what the estimated time for the
current transaction he requested and while the transaction is in progress,
how much time is elapsed for the transaction as a fraction of the total
estimated time at a particular instance, by dynamically estimating the time
for the transaction at that instance.

I got to know how Prostgre estimates the cost for a particular operation.
Does estimated cost means the estimated time to evaluate an operation in the
context of Postgre? And also may I know if there is any way to achieve the
requirement I mentioned above, with the Postgre SQL?

I would be very much thankful for your suggestions as I am doing a research
project to implement a mechanism to achieve the above mentioned task.

Thank you.

regards,
Hasini.