Documentation is seen as an inherently qualitative activity and as such most people think that it can't be measured. If you can't measure something, you can't compare it to other things, even of the same type. If you can't compare it, you can't do any quantitative analysis on it. If you can't do any quantitative analysis on it, you can't put a standardised value on it. If you can't put a value on something, it's a cost.
Personally, I'm of the opinion that at least some of the value can be fairly easily translated into measurable figures, but that's not much good if you can't enunciate exactly what the value of documentation is in the first place. So rather than think of it in terms of "value", which can be quite a vague and nebulous term, try breaking the value down into a list of discrete, individual benefits. Once you have this list you'll see that some benefits are more amenable to quantitative analysis than others and you can focus your attention on measuring the value of those benefits, rather than the more qualitative ones.
You can then use this to help convince the powers-that-be to invest more in documentation.
A starting list might include some or all of the following:
- Helps users understand and use the product, thus increasing customer satisfaction;
- Prevents customers looking to another provider for functionality you already offer;
- Lowers the number of help desk calls;
- Reduces time spent by help desk looking for answers;
- Removes burden of documentation from development;
- Validates product design and usability;
- Removes one aspect of technical debt;
- Provides training material for new starters;
- Assists trainers when producing user training material;
- Prevents single points of failure where information is only retained in people's heads;
- Improves organisational memory;
- Increases internal information flow and reduces staff frustration;
- Provides information for partners and independents to further develop your products and create an ecosystem around them;
- Demonstrates that the company cares about quality;
- Highlights the company as mature and professional;
- Increases IP value of the company;
- Keeps the company compliant with legal/regulatory requirements and audited standards for certifications (like CMMI and ISO);
- Provide a coherent narrative across product suites;
- Provides terminology guidelines/glossary that allow all areas of the business to use a consistent, common voice when talking about products;
- Drives sales by showing customers they'll have the information they need to fix their own problems;
- Provides Marketing and Sales with easy "compare and contrast" information about the product's strengths.
In other words, documentation acts as an internal multiplier to increase efficiencies, lower costs, improve customer satisfaction, and help make sales. Why WOULDN'T you invest in it?
If there is something missing from this list, feel free to add it by leaving a comment!