This is a large topic, but I can recommend the book Million Dollar Consulting by Alan Weiss as a good starting point.
More immediately, you might ask whoever is buying your services for some insight into how your tasks fit into their business goals and the ROI they expect to get from the $ they spend on you. Someone at their org should have this. If not, you get to help them develop such a model which will likely be favorable to you.
Note that you will have to work with the business side to make value-based pricing work at all.
A few books that helped me keep my soul and be productive in consulting:
Crack my head open for re-flashing
- An open heart - Dalai Lama
- Love is the Killer App - Tim Sanders
- The Man who mistook his Job for a life
Consulting:
- High Impact Consulting - This book taught me to only do tiny but really painful items for new customers to build momentum. By getting short term, high impact/pain point items out of the way, it clears the table to say "This consultant has learnt my business and why we do things the way we do". Big projects land themselves. I call it dating before I get married.
More immediately, you might ask whoever is buying your services for some insight into how your tasks fit into their business goals and the ROI they expect to get from the $ they spend on you. Someone at their org should have this. If not, you get to help them develop such a model which will likely be favorable to you.
Note that you will have to work with the business side to make value-based pricing work at all.