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It seems that they are going to pay for your consultancy. In this case, you can evaluate it based on your experience and their IT infrastructure cost. Per hour rate is inaccurate.
In case you may need also to go over the company to investigate their situations and propose updates, charge them hourly plus something more.

What do you think ?

DR can be simple or detailed depending on what level they expect or require. Most simple DR pertains to Hardware Failures. If they expect DR for Software Failure and corruption, human error this elavates the level of complexity and understanding needed.

No matter how good the DR document it is useless if there is no-one on-site that understands it unless they call in an outsider to perform the DR when it happens. To what level do you go to?

Will you then need to TEST the DR to prove it works ( out of hours )

I would charge for a site survey first and then work out an amount to complete the DR whether it be an hourly rate to no more than a certain figure or a quoted full amount.

It sounds more like they need a basic DR along with written procedures and how to perform certain system administrative functions.

There are 2 things that will make this decision difficult for you. 1)How long is it going to take you 2)Exactly how much information do they want.
If you can talk to them and find out exactly what they want then you are in good shape. It is more likely that they don't know exactly what they need, so they are hiring you. In that case you will need to define what information you will document. In either case you need to define a specific way to define when your documentation is done, with specific criteria that can be checked off a list. Otherwise you will have "scope creap", where they keep asking you to do more without being paid for it, or they will claim you didn't do a great job because you didn't account for everything that could possible go wrong (which is nearly impossible.)
As far as quoting them a cost, here is my idea.
Quote them 100 units per hour with a 5 hour minimum, to be paid up front.
Then figure how much time you think it will take to document it the way they want (be honest with yourself, how quickly can you get it done?). Take that X 2 then add 30% (I know it will seem like a lot of time, but there are always hidden things that will come up and cost you more time than you can see right now.) Quote this amount as your maximum time to document as you agreed. The remainder of the bill should be due upon delivery of the completed documentation.
Then by all means put in a stipulation for additional requests, or changes to the amount of information they want documented.
Then, if at all possible, get the job done in less time than the max you quoted. Be sure it looks neat and professional, and is easy to read. Give a non-technical description of each item/process you document so that even the business managers can read it. They will appreciate it and be much more willing to hire you again in the future.
For some information on documenting networks go to NetworkDNA.org . They have templates and links that may be of interest to you.