Thanks to various forum posters in a now somewhat ironically named thread, we have solved yet another numerical mystery. If you want to know how much imagination you will earn per day from the rock simply by existing, it's equal to your qurazy quoin value. Of course usually when you pick up a quoin you'll be getting a 20% bonus from full mood, so you'll have to divide that by 1.2.
According to information from the developers, we know that this is 20% of the maximum you can earn from people visiting your street., so you could earn up to five times the base amount in extra imagination (so up to six times you qurazy quoin value per day).
Also thanks to this discussion we know how to compute that value for levels 22 and up. If you happen to have glitch with a street no one visits, you get the same amount of imagination from the rock every game day, and you are below level 22, please post your level and the amount you get. Just a couple more data points and we'll probably be able to put together the entire chart.