Its not that high.
Assuming 1 million tonnes per 100m, at the given average grades. We get about 400 million $ per 100m. But then the problem is we'll only be mining this resource +/-15 years down the line. So then you've got to include possible inflation/dilution/investments... to name a few.
In the end, its about 200million$ per 100 meter. max. This is what you add to the NPV/IRR... Its still important, as it extends the lifetime of the mine, which inherently will finance some more exploration. But I wouldn't say its alot of cash in the kitty added.
But maybe I'm wrong. Could anyone verify this. Geoprof has also mentioned the deep resources now being unearthed (unless new and huge compared to the rest) wont add much to the IRR/NPV, this was confirmed by Hoov.