You got the idea, but it's off. The S/D forumla is like this.
Y=mx+b.
THE M of this line is +.1 for everything below 15 and -.05 for everything over 15.
At a price of 20, you get .75 regen of prosperity per day. At 15, you get Prosperity regen as shown, per day. At 5, you get 2 times prosperity shown regen per day.
Example:
Prosperity is 200
At 5g per good, you gain 400 S/D per day
At 7g, you gain360 S/D per day
15, you get 200
20 you get 150
25 you get 100