How valuable is product documentation?

We received this email from a Documentation Manager, yesterday:

"As times are getting tougher, we've been challenged with the age old question of "how valuable is product documentation?" - e.g. prove your worth basically!

Certainly, we know that for a product to be marketable and successful, it needs documentation to support the end user.
However, I am currently trying to gather more information specifically about GUI product online help:
1. How often is it really used? I know every product is different so usage will vary - but in general how do user's feel about it?
2. Do user's require it to be context sensitive - or can an e-support Center with all documentation available for searching suffice?
3. What is the current trend for online help structure? Scenario based instructions, quick snippet videos, etc.

Ideally, I'm trying to gain insight into the ROI for documentation efforts spent. If you have any information from past research on the value of documentation, I'd be really grateful if you could share it.

So how would you respond to this challenge?