Rough estimate? If it is a paid directory I would say somewhere around 1-2K I have seen PR4 directories going for 500-650.
A directory valued on PR worth nothing, zero, nada... Today you have PR by the next update you might not have it at all which will make it worthless according to your evaluation criteria. The true value of a directory is in traffic, revenue, quality content and submissions, design, backlinks, etc...
Buying and selling digital goods based on PR is pretty bad. This is exactly why Google has been penalising web directories.
I think The Directories; *have high PR *have low Alexa rank *have daily paid submissions worth much more.
If the rank was maintained for a year plus and not a dropped domain it would go for quite a bit. That said, earnings would be more important in determining the value.