"Doesn't dokupartner need a great deal of time on our premises in order to get to know the functions of our product and how to operate it?" And isn't the indispensable proofreading of drafts a cost factor as well?"

This is not the case. Experience shows that on average we need 12 % of all the time required for creating documentation for this work.
Please remember that you also incur proofreading costs if you create your own documentation.