To a lot of people, acquiring something for free is in itself a reward worth more than the thing itself. People most often associate a higher value for something that's "free". But this it true as long as it's tangible - i.e. it's a "thing".
On the other hand, if you offer your opinion for free, it is taken as just that .... Many years ago, I learned that doing pro-bono consulting as way to get engaged with a client and then grow it into a consulting contract later just doesn't work. A consulting contract would only happen if the client sees value in your services. Once you've offered it for free, the value perception of the service is almost entirely washed out and hence it's almost impossible to recreate it to a point that it's worth paying for.
Lesson learned: Always combine your pro-bono consulting with a free "thing"! Make sure there is a deliverable at the end where your value is captured. If it's a report or presentation, deliver it in a nicely bound printed on good quality paper. If it's something that required a lot of time and heavy thinking, make it large and heavy. In either case, present it with a big final bang (invite board members, make several copies, etc.).
Once your have the free "thing" delivered, it's much easier to move to the contract negotiation.