Hello,
I think you can get p1 points for each stone in the zone. In my point of view the bonus points are p2, p3 and p4. So if you build a wall you get 3 times p1 and if the wall is valid, you get p2 added to your points. For example: p1 is 2 points and p2 is 4 points, you get for a wall 3 x 2 + 4 = 10 points. If you build a wall with only two stones you get 2 x 2 = 4 points. The same for the other buildings.
You can find the zone plane in Figure 6 on page 11.
BR
Michael