PC Coffer of Whispers

How much is a Coffer of Whispers worth?

Well...
I would say as an Armbrace sells at 29 - 30e at the moment, and there are 15 gemsets to an Armbrace, a coffer should set you back about 2e (13-15k) though with GW2 around teh corner, the final push for Armbraces would likely mean there is more demand for a gemset than a coffer.
Hope that is of use to you.

Hmmm, then how much do you think a gem would sell for individually?

A coffer of whispers sells anywhere from 2e-3e or 15k-18k depending on the situation.