Your author rewards are worth money
Calculate your hourly rate of compensation for making your posts. Your author rewards are worth money. If you calculate the amount of time, all of it, it takes to put together your posts in a typical week and divide your weekly author rewards by the number of hours you put into making the posts, you will get your hourly pay. Let's call that X quatloos per hour.
Your time is worth money
Estimate your hourly compensation if you did overtime in your job or took a second job. Your time is worth money. How much would you be able to earn if you took on a second job or if you put in more hours in your main job? Let's call that Y quatloos per hour. Let's assume there was no enjoyment or other type of subjective value (calling, future potential etc.) in that extra job and that you wouldn't do a minute of it if you didn't get paid.
How to calculate what proportion of your motivation is enjoyment?
Let's call that proportion E.
By spending time posting to Hive, the amount of money you're throwing away as an opportunity cost is simply the difference between what you'd earn by doing a job you could be doing but only for the money, that is Y-X. The gain you're making by allotting that time to post on Hive is X.
Then E = (Y-X)/Y because (Y-X) is the amount of money you're willing to forego in exchange for the opportunity to post on Hive.
For example, if you put in ten hours a week making your posts and you earn $50 a week, your hourly compensation is $5. Let's assume you live in a first world country and you can find a second job that paid $20 per hour or that your regular job pays that much if you did more hours (assuming they were available) after taxes and the job were such that you wouldn't do a minute of it if you didn't get paid.
Then E = (20-5)/20 = 75%.
Does this make sense? :)
Note that if you become less likely to invest in HIVE after cutting down on your time posting to Hive and dedicating that time to earning more money, then you have not been rational about the monetary value of your time. The assumption here is that you have been involved in the community and you have gained a level of knowledge that allows you to make an informed decision to invest or not invest in HIVE.