Dear Employer.
Let me explain the nature of our relationship.
I have knowledge, skills and time to sell you, you have money to buy my time and use my knowledge and skills during that time.
We have an agreement where I work 0800 to 1700 every normal work day for a negotiated rate.
During that time you are the one to tell me what my tasks are and reap the benefits of their completion.
Outside of that time I decide my own tasks to complete and reap my own rewards for completing them.
If you want to gain access to data I produce outside normal work hours the time it takes to produce said data is me working for you. (Remember that this is a task you have requested I perform).
This means that this is a new business relationship you want to engage with me.
To cut to the chase, how much is my proprietary data worth to you?